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Volcanic Islands by Charles Darwin

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colours, interspersed with a few volcanic particles. At the depth of a few
feet, these are found cemented together into stone, of which the softer
varieties are used for building; there are other varieties, both coarse and
fine-grained, too hard for this purpose: and I saw one mass divided into
even layers half an inch in thickness, which were so compact that when
struck with a hammer they rang like flint. It is believed by the
inhabitants, that the particles become united in the course of a single
year. The union is effected by calcareous matter; and in the most compact
varieties, each rounded particle of shell and volcanic rock can be
distinctly seen to be enveloped in a husk of pellucid carbonate of lime.
Extremely few perfect shells are embedded in these agglutinated masses; and
I have examined even a large fragment under a microscope, without being
able to discover the least vestige of striae or other marks of external
form: this shows how long each particle must have been rolled about, before
its turn came to be embedded and cemented. (The eggs of the turtle being
buried by the parent, sometimes become enclosed in the solid rock. Mr.
Lyell has given a figure ("Principles of Geology" book 3 chapter 17) of
some eggs, containing the bones of young turtles, found thus entombed.) One
of the most compact varieties, when placed in acid, was entirely dissolved,
with the exception of some flocculent animal matter; its specific gravity
was 2.63. The specific gravity of ordinary limestone varies from 2.6 to
2.75; pure Carrara marble was found by Sir H. De la Beche to be 2.7.
("Researches in Theoretical Geology" page 12.) It is remarkable that these
rocks of Ascension, formed close to the surface, should be nearly as
compact as marble, which has undergone the action of heat and pressure in
the plutonic regions.

The great accumulation of loose calcareous particles, lying on the beach
near the Settlement, commences in the month of October, moving towards the
S.W., which, as I was informed by Lieutenant Evans, is caused by a change
in the prevailing direction of the currents. At this period the tidal
rocks, at the S.W. end of the beach, where the calcareous sand is
accumulating, and round which the currents sweep, become gradually coated
with a calcareous incrustation, half an inch in thickness. It is quite
white, compact, with some parts slightly spathose, and is firmly attached
to the rock. After a short time it gradually disappears, being either
redissolved, when the water is less charged with lime, or more probably is
mechanically abraded. Lieutenant Evans has observed these facts, during the
six years he has resided at Ascension. The incrustation varies in thickness
in different years: in 1831 it was unusually thick. When I was there in
July, there was no remnant of the incrustation; but on a point of basalt,
from which the quarrymen had lately removed a mass of the calcareous
freestone, the incrustation was perfectly preserved. Considering the
position of the tidal-rocks, and the period at which they become coated,
there can be no doubt that the movement and disturbance of the vast
accumulation of calcareous particles, many of them being partially
agglutinated together, cause the waves of the sea to be so highly charged
with carbonate of lime, that they deposit it on the first objects against
which they impinge. I have been informed by Lieutenant Holland, R.N., that
this incrustation is formed on many parts of the coast, on most of which, I
believe, there are likewise great masses of comminuted shells.


tidal-rocks at Ascension.)

In many respects this is a singular deposit; it coats throughout the year
the tidal volcanic rocks, that project from the beaches composed of broken
shells. Its general appearance is well represented in Figure 5; but the
fronds or discs, of which it is composed, are generally so closely crowded
together as to touch. These fronds have their sinuous edges finely
crenulated, and they project over their pedestals or supports; their upper
surfaces are either slightly concave, or slightly convex; they are highly
polished, and of a dark grey or jet black colour; their form is irregular,
generally circular, and from the tenth of an inch to one inch and a half in
diameter; their thickness, or amount of their projection from the rock on
which they stand, varies much, about a quarter of an inch being perhaps
most usual. The fronds occasionally become more and more convex, until they
pass into botryoidal masses with their summits fissured; when in this
state, they are glossy and of an intense black, so as to resemble some
fused metallic substance. I have shown the incrustation, both in this
latter and in its ordinary state to several geologists, but not one could
conjecture its origin, except that perhaps it was of volcanic nature!

The substance forming the fronds has a very compact and often almost
crystalline fracture; the edges being translucent, and hard enough easily
to scratch calcareous spar. Under the blowpipe it immediately becomes
white, and emits a strong animal odour, like that from fresh shells. It is
chiefly composed of carbonate of lime; when placed in muriatic acid it
froths much, leaving a residue of sulphate of lime, and of an oxide of
iron, together with a black powder, which is not soluble in heated acids.
This latter substance seems to be carbonaceous, and is evidently the
colouring matter. The sulphate of lime is extraneous, and occurs in
distinct, excessively minute, lamellar plates, studded on the surface of
the fronds, and embedded between the fine layers of which they are
composed; when a fragment is heated in the blowpipe, these lamellae are
immediately rendered visible. The original outline of the fronds may often
be traced, either to a minute particle of shell fixed in a crevice of the
rock, or to several cemented together; these first become deeply corroded,
by the dissolving power of the waves, into sharp ridges, and then are
coated with successive layers of the glossy, grey, calcareous incrustation.
The inequalities of the primary support affect the outline of every
successive layer, in the same manner as may often be seen in bezoar-stones,
when an object like a nail forms the centre of aggregation. The crenulated
edges, however, of the frond appear to be due to the corroding power of the
surf on its own deposit, alternating with fresh depositions. On some smooth
basaltic rocks on the coast of St. Jago, I found an exceedingly thin layer
of brown calcareous matter, which under a lens presented a miniature
likeness of the crenulated and polished fronds of Ascension; in this case a
basis was not afforded by any projecting extraneous particles. Although the
incrustation at Ascension is persistent throughout the year; yet from the
abraded appearance of some parts, and from the fresh appearance of other
parts, the whole seems to undergo a round of decay and renovation, due
probably to changes in the form of the shifting beach, and consequently in
the action of the breakers: hence probably it is, that the incrustation
never acquires a great thickness. Considering the position of the encrusted
rocks in the midst of the calcareous beach, together with its composition,
I think there can be no doubt that its origin is due to the dissolution and
subsequent deposition of the matter composing the rounded particles of
shells and corals. (The selenite, as I have remarked is extraneous, and
must have been derived from the sea-water. It is an interesting
circumstance thus to find the waves of the ocean, sufficiently charged with
sulphate of lime, to deposit it on the rocks, against which they dash every
tide. Dr. Webster has described ("Voyage of the 'Chanticleer'" volume 2
page 319) beds of gypsum and salt, as much as two feet in thickness, left
by the evaporation of the spray on the rocks on the windward coast.
Beautiful stalactites of selenite, resembling in form those of carbonate of
lime, are formed near these beds. Amorphous masses of gypsum, also, occur
in caverns in the interior of the island; and at Cross Hill (an old crater)
I saw a considerable quantity of salt oozing from a pile of scoriae. In
these latter cases, the salt and gypsum appear to be volcanic products.)
From this source it derives its animal matter, which is evidently the
colouring principle. The nature of the deposit, in its incipient stage, can
often be well seen upon a fragment of white shell, when jammed between two
of the fronds; it then appears exactly like the thinnest wash of a pale
grey varnish. Its darkness varies a little, but the jet blackness of some
of the fronds and of the botryoidal masses seems due to the translucency of
the successive grey layers. There is, however, this singular circumstance,
that when deposited on the under side of ledges of rock or in fissures, it
appears always to be of a pale, pearly grey colour, even when of
considerable thickness: hence one is led to suppose, that an abundance of
light is necessary to the development of the dark colour, in the same
manner as seems to be the case with the upper and exposed surfaces of the
shells of living mollusca, which are always dark, compared with their under
surfaces and with the parts habitually covered by the mantle of the animal.
In this circumstance,--in the immediate loss of colour and in the odour
emitted under the blowpipe,--in the degree of hardness and translucency of
the edges,--and in the beautiful polish of the surface (From the fact
described in my "Journal of Researches" of a coating of oxide of iron,
deposited by a streamlet on the rocks in its bed (like a nearly similar
coating at the great cataracts of the Orinoco and Nile), becoming finely
polished where the surf acts, I presume that the surf in this instance,
also, is the polishing agent.), rivalling when in a fresh state that of the
finest Oliva, there is a striking analogy between this inorganic
incrustation and the shells of living molluscous animals. (In the section
descriptive of St. Paul's Rocks, I have described a glossy, pearly
substance, which coats the rocks, and an allied stalactitical incrustation
from Ascension, the crust of which resembles the enamel of teeth, but is
hard enough to scratch plate-glass. Both these substances contain animal
matter, and seem to have been derived from water in filtering through
birds' dung.) This appears to me to be an interesting physiological fact.
(Mr. Horner and Sir David Brewster have described "Philosophical
Transactions" 1836 page 65 a singular "artificial substance, resembling
shell." It is deposited in fine, transparent, highly polished, brown-
coloured laminae, possessing peculiar optical properties, on the inside of
a vessel, in which cloth, first prepared with glue and then with lime, is
made to revolve rapidly in water. It is much softer, more transparent, and
contains more animal matter, than the natural incrustation at Ascension;
but we here again see the strong tendency which carbonate of lime and
animal matter evince to form a solid substance allied to shell.)


These beds occur within the trachytic district, at the western base of
Green Mountain, under which they dip at a high inclination. They are only
partially exposed, being covered up by modern ejections; from this cause, I
was unable to trace their junction with the trachyte, or to discover
whether they had flowed as a stream of lava, or had been injected amidst
the overlying strata. There are three principal beds of obsidian, of which
the thickest forms the base of the section. The alternating stony layers
appear to me eminently curious, and shall be first described, and
afterwards their passage into the obsidian. They have an extremely
diversified appearance; five principal varieties may be noticed, but these
insensibly blend into each other by endless gradations.


A pale grey, irregularly and coarsely laminated (This term is open to some
misinterpretation, as it may be applied both to rocks divided into laminae
of exactly the same composition, and to layers firmly attached to each
other, with no fissile tendency, but composed of different minerals, or of
different shades of colour. The term "laminated," in this chapter, is
applied in these latter senses; where a homogeneous rock splits, as in the
former sense, in a given direction, like clay-slate, I have used the term
"fissile."), harsh-feeling rock, resembling clay-slate which has been in
contact with a trap-dike, and with a fracture of about the same degree of
crystalline structure. This rock, as well as the following varieties,
easily fuses into a pale glass. The greater part is honeycombed with
irregular, angular, cavities, so that the whole has a curious appearance,
and some fragments resemble in a remarkable manner silicified logs of
decayed wood. This variety, especially where more compact, is often marked
with thin whitish streaks, which are either straight or wrap round, one
behind the other, the elongated carious hollows.


A bluish grey or pale brown, compact, heavy, homogeneous stone, with an
angular, uneven, earthy fracture; viewed, however, under a lens of high
power, the fracture is seen to be distinctly crystalline, and even separate
minerals can be distinguished.


A stone of the same kind with the last, but streaked with numerous,
parallel, slightly tortuous, white lines of the thickness of hairs. These
white lines are more crystalline than the parts between them; and the stone
splits along them: they frequently expand into exceedingly thin cavities,
which are often only just perceptible with a lens. The matter forming the
white lines becomes better crystallised in these cavities, and Professor
Miller was fortunate enough, after several trials, to ascertain that the
white crystals, which are the largest, were of quartz (Professor Miller
informs me that the crystals which he measured had the faces P, z, m of the
figure (147) given by Haidinger in his Translation of Mohs; and he adds,
that it is remarkable, that none of them had the slightest trace of faces r
of the regular six-sided prism.), and that the minute green transparent
needles were augite, or, as they would more generally be called, diopside:
besides these crystals, there are some minute, dark specks without a trace
of crystalline, and some fine, white, granular, crystalline matter which is
probably feldspar. Minute fragments of this rock are easily fusible.


A compact crystalline rock, banded in straight lines with innumerable
layers of white and grey shades of colour, varying in width from the
thirtieth to the two-hundredth of an inch; these layers seem to be composed
chiefly of feldspar, and they contain numerous perfect crystals of glassy
feldspar, which are placed lengthways; they are also thickly studded with
microscopically minute, amorphous, black specks, which are placed in rows,
either standing separately, or more frequently united, two or three or
several together, into black lines, thinner than a hair. When a small
fragment is heated in the blowpipe, the black specks are easily fused into
black brilliant beads, which become magnetic,--characters that apply to no
common mineral except hornblende or augite. With the black specks there are
mingled some others of a red colour, which are magnetic before being
heated, and no doubt are oxide of iron. Round two little cavities, in a
specimen of this variety, I found the black specks aggregated into minute
crystals, appearing like those of augite or hornblende, but too dull and
small to be measured by the goniometer; in the specimen, also, I could
distinguish amidst the crystalline feldspar, grains, which had the aspect
of quartz. By trying with a parallel ruler, I found that the thin grey
layers and the black hair-like lines were absolutely straight and parallel
to each other. It is impossible to trace the gradation from the homogeneous
grey rocks to these striped varieties, or indeed the character of the
different layers in the same specimen, without feeling convinced that the
more or less perfect whiteness of the crystalline feldspathic matter
depends on the more or less perfect aggregation of diffused matter, into
the black and red specks of hornblende and oxide of iron.


A compact heavy rock, not laminated, with an irregular, angular, highly
crystalline, fracture; it abounds with distinct crystals of glassy
feldspar, and the crystalline feldspathic base is mottled with a black
mineral, which on the weathered surface is seen to be aggregated into small
crystals, some perfect, but the greater number imperfect. I showed this
specimen to an experienced geologist, and asked him what it was; he
answered, as I think every one else would have done, that it was a
primitive greenstone. The weathered surface, also, of the banded variety in
Figure 4, strikingly resembles a worn fragment of finely laminated gneiss.

These five varieties, with many intermediate ones, pass and repass into
each other. As the compact varieties are quite subordinate to the others,
the whole may be considered as laminated or striped. The laminae, to sum up
their characteristics, are either quite straight, or slightly tortuous, or
convoluted; they are all parallel to each other, and to the intercalating
strata of obsidian; they are generally of extreme thinness; they consist
either of an apparently homogeneous, compact rock, striped with different
shades of grey and brown colours, or of crystalline feldspathic layers in a
more or less perfect state of purity, and of different thicknesses, with
distinct crystals of glassy feldspar placed lengthways, or of very thin
layers chiefly composed of minute crystals of quartz and augite, or
composed of black and red specks of an augitic mineral and of an oxide of
iron, either not crystallised or imperfectly so. After having fully
described the obsidian, I shall return to the subject of the lamination of
rocks of the trachytic series.

The passage of the foregoing beds into the strata of glassy obsidian is
effected in several ways: first, angulo-modular masses of obsidian, both
large and small, abruptly appear disseminated in a slaty, or in an
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
fracture. Secondly, small irregular nodules of the obsidian, either
standing separately, or united into thin layers, seldom more than the tenth
of an inch in thickness, alternate repeatedly with very thin layers of a
feldspathic rock, which is striped with the finest parallel zones of
colour, like an agate, and which sometimes passes into the nature of
pitchstone; the interstices between the nodules of obsidian are generally
filled by soft white matter, resembling pumiceous ashes. Thirdly, the whole
substance of the bounding rock suddenly passes into an angulo-concretionary
mass of obsidian. Such masses (as well as the small nodules) of obsidian
are of a pale green colour, and are generally streaked with different
shades of colour, parallel to the laminae of the surrounding rock; they
likewise generally contain minute white sphaerulites, of which half is
sometimes embedded in a zone of one shade of colour, and half in a zone of
another shade. The obsidian assumes its jet black colour and perfectly
conchoidal fracture, only when in large masses; but even in these, on
careful examination and on holding the specimens in different lights, I
could generally distinguish parallel streaks of different shades of

(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper
ones are externally marked with parallel ridges. The internal radiating
structure of the lower ones, is much too plainly represented.

INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the
natural size.)

One of the commonest transitional rocks deserves in several respects a
further description. It is of a very complicated nature, and consists of
numerous thin, slightly tortuous layers of a pale-coloured feldspathic
stone, often passing into an imperfect pitchstone, alternating with layers
formed of numberless little globules of two varieties of obsidian, and of
two kinds of sphaerulites, embedded in a soft or in a hard pearly base. The
sphaerulites are either white and translucent, or dark brown and opaque;
the former are quite spherical, of small size, and distinctly radiated from
their centre. The dark brown sphaerulites are less perfectly round, and
vary in diameter from the twentieth to the thirtieth of an inch; when
broken they exhibit towards their centres, which are whitish, an obscure
radiating structure; two of them when united sometimes have only one
central point of radiation; there is occasionally a trace of or a hollow
crevice in their centres. They stand either separately, or are united two
or three or many together into irregular groups, or more commonly into
layers, parallel to the stratification of the mass. This union in many
cases is so perfect, that the two sides of the layer thus formed, are quite
even; and these layers, as they become less brown and opaque, cannot be
distinguished from the alternating layers of the pale-coloured feldspathic
stone. The sphaerulites, when not united, are generally compressed in the
plane of the lamination of the mass; and in this same plane, they are often
marked internally, by zones of different shades of colour, and externally
by small ridges and furrows. In the upper part of Figure 6, the
sphaerulites with the parallel ridges and furrows are represented on an
enlarged scale, but they are not well executed; and in the lower part,
their usual manner of grouping is shown. In another specimen, a thin layer
formed of the brown sphaerulites closely united together, intersects, as
represented in Figure 7, a layer of similar composition; and after running
for a short space in a slightly curved line, again intersects it, and
likewise a second layer lying a little way beneath that first intersected.
The small nodules also of obsidian are sometimes externally marked with
ridges and furrows, parallel to the lamination of the mass, but always less
plainly than the sphaerulites. These obsidian nodules are generally
angular, with their edges blunted: they are often impressed with the form
of the adjoining sphaerulites, than which they are always larger; the
separate nodules seldom appear to have drawn each other out by exerting a
mutually attractive force. Had I not found in some cases, a distinct centre
of attraction in these nodules of obsidian, I should have been led to have
considered them as residuary matter, left during the formation of the
pearlstone, in which they are embedded, and of the sphaerulitic globules.

The sphaerulites and the little nodules of obsidian in these rocks so
closely resemble, in general form and structure, concretions in sedimentary
deposits, that one is at once tempted to attribute to them an analogous
origin. They resemble ordinary concretions in the following respects: in
their external form,--in the union of two or three, or of several, into an
irregular mass, or into an even-sided layer,--in the occasional
intersection of one such layer by another, as in the case of chalk-flints,-
-in the presence of two or three kinds of nodules, often close together, in
the same basis,--in their fibrous, radiating structure, with occasional
hollows in their centres,--in the co-existence of a laminary,
concretionary, and radiating structure, as is so well developed in the
concretions of magnesian limestone, described by Professor Sedgwick.
("Geological Transactions" volume 3 part 1 page 37.) Concretions in
sedimentary deposits, it is known, are due to the separation from the
surrounding mass of the whole or part of some mineral substance, and its
aggregation round certain points of attraction. Guided by this fact, I have
endeavoured to discover whether obsidian and the sphaerulites (to which may
be added marekanite and pearlstone, both of them occurring in nodular
concretions in the trachytic series) differ in their constituent parts,
from the minerals generally composing trachytic rocks. It appears from
three analyses, that obsidian contains on an average 76 per cent of silica;
from one analysis, that sphaerulites contain 79.12; from two, that
marekanite contains 79.25; and from two other analyses, that pearlstone
contains 75.62 of silica. (The foregoing analyses are taken from Beudant
"Traite de Mineralogie" tome 2 page 113; and one analysis of obsidian from
Phillips "Mineralogy.") Now, the constituent parts of trachyte, as far as
they can be distinguished consist of feldspar, containing 65.21 of silica;
or of albite, containing 69.09; of hornblende, containing 55.27 (These
analyses are taken from Von Kobell "Grundzuge der Mineralogie" 1838.), and
of oxide of iron: so that the foregoing glassy concretionary substances all
contain a larger proportion of silica than that occurring in ordinary
feldspathic or trachytic rocks. D'Aubuisson ("Traite de Geogn." tome 2 page
535.), also, has remarked on the large proportion of silica compared with
alumina, in six analyses of obsidian and pearlstone given in Brongniart's
"Mineralogy." Hence I conclude, that the foregoing concretions have been
formed by a process of aggregation, strictly analogous to that which takes
place in aqueous deposits, acting chiefly on the silica, but likewise on
some of the other elements of the surrounding mass, and thus producing the
different concretionary varieties. From the well-known effects of rapid
cooling (This is seen in the manufacture of common glass, and in Gregory
Watts's experiments on molten trap; also on the natural surfaces of lava-
streams, and on the side-walls of dikes.) in giving glassiness of texture,
it is probably necessary that the entire mass, in cases like that of
Ascension, should have cooled at a certain rate; but considering the
repeated and complicated alterations of nodules and thin layers of a glassy
texture with other layers quite stony or crystalline, all within the space
of a few feet or even inches, it is hardly possible that they could have
cooled at different rates, and thus have acquired their different textures.

The natural sphaerulites in these rocks very closely resemble those
produced in glass, when slowly cooled. (I do not know whether it is
generally known, that bodies having exactly the same appearance as
sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in an
agate, formed within a cavity in a piece of silicified wood, some little
specks, which were only just visible to the naked eye: these specks, when
placed by him under a lens of high power, presented a beautiful appearance:
they were perfectly circular, and consisted of the finest fibres of a brown
colour, radiating with great exactness from a common centre. These little
radiating stars are occasionally intersected, and portions are quite cut
off by the fine, ribbon-like zones of colour in the agate. In the obsidian
of Ascension, the halves of a sphaerulite often lie in different zones of
colour, but they are not cut off by them, as in the agate.) In some fine
specimens of partially devitrified glass, in the possession of Mr. Stokes,
the sphaerulites are united into straight layers with even sides, parallel
to each other, and to one of the outer surfaces, exactly as in the
obsidian. These layers sometimes interbranch and form loops; but I did not
see any case of actual intersection. They form the passage from the
perfectly glassy portions, to those nearly homogeneous and stony, with only
an obscure concretionary structure. In the same specimen, also,
sphaerulites differing slightly in colour and in structure, occur embedded
close together. Considering these facts, it is some confirmation of the
view above given of the concretionary origin of the obsidian and natural
sphaerulites, to find that M. Dartigues ("Journal de Physique" tome 59 1804
pages 10, 12.), in his curious paper on this subject, attributes the
production of sphaerulites in glass, to the different ingredients obeying
their own laws of attraction and becoming aggregated. He is led to believe
that this takes place, from the difficulty in remelting sphaerulitic glass,
without the whole be first thoroughly pounded and mixed together; and
likewise from the fact, that the change takes place most readily in glass
composed of many ingredients. In confirmation of M. Dartigues' view, I may
remark, that M. Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found
that the sphaerulitic portions of devitrified glass were acted on both by
nitric acid and under the blowpipe, in a different manner from the compact
paste in which they were embedded.


I have been struck with much surprise, how closely the excellent
description of the obsidian rocks of Hungary, given by Beudant ("Voyage en
Hongrie" tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, 371,
377, 381.), and that by Humboldt, of the same formation in Mexico and Peru
("Essai Geognostique" pages 176, 326, 328.), and likewise the descriptions
given by several authors (P. Scrope "Geological Transactions" volume 2
second series page 195. Consult also Dolomieu "Voyage aux Isles Lipari" and
D'Aubuisson "Traite de Geogn." tome 2 page 534.) of the trachytic regions
in the Italian islands, agree with my observations at Ascension. Many
passages might have been transferred without alteration from the works of
the above authors, and would have been applicable to this island. They all
agree in the laminated and stratified character of the whole series; and
Humboldt speaks of some of the beds of obsidian being ribboned like jasper.
(In Mr. Stokes' fine collection of obsidians from Mexico, I observe that
the sphaerulites are generally much larger than those of Ascension; they
are generally white, opaque, and are united into distinct layers: there are
many singular varieties, different from any at Ascension. The obsidians are
finely zoned, in quite straight or curved lines, with exceedingly slight
differences of tint, of cellularity, and of more or less perfect degrees of
glassiness. Tracing some of the less perfectly glassy zones, they are seen
to become studded with minute white sphaerulites, which become more and
more numerous, until at last they unite and form a distinct layer: on the
other hand, at Ascension, only the brown sphaerulites unite and form
layers; the white ones always being irregularly disseminated. Some
specimens at the Geological Society, said to belong to an obsidian
formation from Mexico, have an earthy fracture, and are divided in the
finest parallel laminae, by specks of a black mineral, like the augitic or
hornblendic specks in the rocks at Ascension.) They all agree in the
nodular or concretionary character of the obsidian, and of the passage of
these nodules into layers. They all refer to the repeated alterations,
often in undulatory planes, of glassy, pearly, stony, and crystalline
layers: the crystalline layers, however, seem to be much more perfectly
developed at Ascension, than in the above-named countries. Humboldt
compares some of the stony beds, when viewed from a distance, to strata of
a schistose sandstone. Sphaerulites are described as occurring abundantly
in all cases; and they everywhere seem to mark the passage, from the
perfectly glassy to the stony and crystalline beds. Beudant's account
(Beudant "Voyage" tome 3 page 373.) of his "perlite lithoide globulaire" in
every, even the most trifling particular, might have been written for the
little brown sphaerulitic globules of the rocks of Ascension.

From the close similarity in so many respects, between the obsidian
formations of Hungary, Mexico, Peru, and of some of the Italian islands,
with that of Ascension, I can hardly doubt that in all these cases, the
obsidian and the sphaerulites owe their origin to a concretionary
aggregation of the silica, and of some of the other constituent elements,
taking place whilst the liquified mass cooled at a certain required rate.
It is, however, well-known, that in several places, obsidian has flowed in
streams like lava; for instance, at Teneriffe, at the Lipari Islands, and
at Iceland. (For Teneriffe see von Buch "Descript. des Isles Canaries"
pages 184 and 190; for the Lipari Islands see Dolomieu "Voyage" page 34;
for Iceland see Mackenzie "Travels" page 369.) In these cases, the
superficial parts are the most perfectly glassy, the obsidian passing at
the depth of a few feet into an opaque stone. In an analysis by Vauquelin
of a specimen of obsidian from Hecla, which probably flowed as lava, the
proportion of silica is nearly the same as in the nodular or concretionary
obsidian from Mexico. It would be interesting to ascertain, whether the
opaque interior portions and the superficial glassy coating contained the
same proportional constituent parts: we know from M. Dufrenoy ("Memoires
pour servir a une Descript. Geolog. de la France" tome 4 page 371.) that
the exterior and interior parts of the same stream of lava sometimes differ
considerably in their composition. Even should the whole body of the stream
of obsidian turn out to be similarly composed with nodular obsidian, it
would only be necessary, in accordance with the foregoing facts, to suppose
that lava in these instances had been erupted with its ingredients mixed in
the same proportion, as in the concretionary obsidian.


We have seen that, in several and widely distant countries, the strata
alternating with beds of obsidian, are highly laminated. The nodules, also,
both large and small, of the obsidian, are zoned with different shades of
colour; and I have seen a specimen from Mexico in Mr. Stokes' collection,
with its external surface weathered (MacCulloch states "Classification of
Rocks" page 531 that the exposed surfaces of the pitchstone dikes in Arran
are furrowed "with undulating lines, resembling certain varieties of
marbled paper, and which evidently result from some corresponding
difference of laminar structure.") into ridges and furrows, corresponding
with the zones of different degrees of glassiness: Humboldt ("Personal
Narrative" volume 1 page 222.), moreover, found on the Peak of Teneriffe, a
stream of obsidian divided by very thin, alternating, layers of pumice.
Many other lavas of the feldspathic series are laminated; thus, masses of
common trachyte at Ascension are divided by fine earthy lines, along which
the rock splits, separating thin layers of slightly different shades of
colour; the greater number, also, of the embedded crystals of glassy
feldspar are placed lengthways in the same direction. Mr. P. Scrope
("Geological Transactions" volume 2 second series page 195.) has described
a remarkable columnar trachyte in the Panza Islands, which seems to have
been injected into an overlying mass of trachytic conglomerate: it is
striped with zones, often of extreme tenuity, of different textures and
colours; the harder and darker zones appearing to contain a larger
proportion of silica. In another part of the island, there are layers of
pearlstone and pitchstone, which in many respects resemble those of
Ascension. The zones in the columnar trachyte are generally contorted; they
extend uninterruptedly for a great length in a vertical direction, and
apparently parallel to the walls of the dike-like mass. Von Buch
("Description des Iles Canaries" page 184.) has described at Teneriffe, a
stream of lava containing innumerable thin, plate-like crystals of
feldspar, which are arranged like white threads, one behind the other, and
which mostly follow the same direction. Dolomieu ("Voyage aux Isles de
Lipari" pages 35 and 85.) also states, that the grey lavas of the modern
cone of Vulcano, which have a vitreous texture, are streaked with parallel
white lines: he further describes a solid pumice-stone which possesses a
fissile structure, like that of certain micaceous schists. Phonolite, which
I may observe is often, if not always, an injected rock, also, often has a
fissile structure; this is generally due to the parallel position of the
embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems
to be nearly independent of their presence. (In this case, and in that of
the fissile pumice-stone, the structure is very different from that in the
foregoing cases, where the laminae consist of alternate layers of different
composition or texture. In some sedimentary formations, however, which
apparently are homogeneous and fissile, as in glossy clay-slate, there is
reason to believe, according to D'Aubuisson, that the laminae are really
due to excessively thin, alternating, layers of mica.) From these facts we
see, that various rocks of the feldspathic series have either a laminated
or fissile structure, and that it occurs both in masses which have injected
into overlying strata, and in others which have flowed as streams of lava.

The laminae of the beds, alternating with the obsidian at Ascension, dip at
a high angle under the mountain, at the base of which they are situated;
and they do not appear as if they had been inclined by violence. A high
inclination is common to these beds in Mexico, Peru, and in some of the
Italian islands (See Phillips "Mineralogy" for the Italian Islands page
136. For Mexico and Peru see Humboldt "Essai Geognostique." Mr. Edwards
also describes the high inclination of the obsidian rocks of the Cerro del
Navaja in Mexico in the "Proc. of the Geolog. Soc." June 1838.): on the
other hand, in Hungary, the layers are horizontal; the laminae, also, of
some of the lava-streams above referred to, as far as I can understand the
descriptions given of them, appear to be highly inclined or vertical. I
doubt whether in any of these cases, the laminae have been tilted into
their present position; and in some instances, as in that of the trachyte
described by Mr. Scrope, it is almost certain that they have been
originally formed with a high inclination. In many of these cases, there is
evidence that the mass of liquified rock has moved in the direction of the
laminae. At Ascension, many of the air-cells have a drawn out appearance,
and are crossed by coarse semi-glassy fibres, in the direction of the
laminae; and some of the layers, separating the sphaerulitic globules, have
a scored appearance, as if produced by the grating of the globules. I have
seen a specimen of zoned obsidian from Mexico, in Mr. Stokes' collection,
with the surfaces of the best-defined layers streaked or furrowed with
parallel lines; and these lines or streaks precisely resembled those,
produced on the surface of a mass of artificial glass by its having been
poured out of a vessel. Humboldt, also, has described little cavities,
which he compares to the tails of comets, behind sphaerulites in laminated
obsidian rocks from Mexico, and Mr. Scrope has described other cavities
behind fragments embedded in his laminated trachyte, and which he supposes
to have been produced during the movement of the mass. ("Geological
Transactions" volume 2 second series page 200 etc. These embedded
fragments, in some instances, consist of the laminated trachyte broken off
and "enveloped in those parts, which still remained liquid." Beudant, also,
frequently refers in his great work on "Hungary" tome 3 page 386, to
trachytic rocks, irregularly spotted with fragments of the same varieties,
which in other parts form the parallel ribbons. In these cases, we must
suppose, that after part of the molten mass had assumed a laminated
structure, a fresh irruption of lava broke up the mass, and involved
fragments, and that subsequently the whole became relaminated.) From such
facts, most authors have attributed the lamination of these volcanic rocks
to their movement whilst liquified. Although it is easy to perceive, why
each separate air-cell, or each fibre in pumice-stone (Dolomieu "Voyage"
page 64.), should be drawn out in the direction of the moving mass; it is
by no means at first obvious why such air-cells and fibres should be
arranged by the movement, in the same planes, in laminae absolutely
straight and parallel to each other, and often of extreme tenuity; and
still less obvious is it, why such layers should come to be of slightly
different composition and of different textures.

In endeavouring to make out the cause of the lamination of these igneous
feldspathic rocks, let us return to the facts so minutely described at
Ascension. We there see, that some of the thinnest layers are chiefly
formed by numerous, exceedingly minute, though perfect, crystals of
different minerals; that other layers are formed by the union of different
kinds of concretionary globules, and that the layers thus formed, often
cannot be distinguished from the ordinary feldspathic and pitchstone
layers, composing a large portion of the entire mass. The fibrous radiating
structure of the sphaerulites seems, judging from many analogous cases, to
connect the concretionary and crystalline forces: the separate crystals,
also, of feldspar all lie in the same parallel planes. (The formation,
indeed, of a large crystal of any mineral in a rock of mixed composition
implies an aggregation of the requisite atoms, allied to concretionary
action. The cause of the crystals of feldspar in these rocks of Ascension,
being all placed lengthways, is probably the same with that which elongates
and flattens all the brown sphaerulitic globules (which behave like
feldspar under the blowpipe) in this same direction.) These allied forces,
therefore, have played an important part in the lamination of the mass, but
they cannot be considered the primary force; for the several kinds of
nodules, both the smallest and largest, are internally zoned with
excessively fine shades of colour, parallel to the lamination of the whole;
and many of them are, also, externally marked in the same direction with
parallel ridges and furrows, which have not been produced by weathering.

Some of the finest streaks of colour in the stony layers, alternating with
the obsidian, can be distinctly seen to be due to an incipient
crystallisation of the constituent minerals. The extent to which the
minerals have crystallised can, also, be distinctly seen to be connected
with the greater or less size, and with the number, of the minute,
flattened, crenulated air-cavities or fissures. Numerous facts, as in the
case of geodes, and of cavities in silicified wood, in primary rocks, and
in veins, show that crystallisation is much favoured by space. Hence, I
conclude, that, if in a mass of cooling volcanic rock, any cause produced
in parallel planes a number of minute fissures or zones of less tension
(which from the pent-up vapours would often be expanded into crenulated
air-cavities), the crystallisation of the constituent parts, and probably
the formation of concretions, would be superinduced or much favoured in
such planes; and thus, a laminated structure of the kind we are here
considering would be generated.

That some cause does produce parallel zones of less tension in volcanic
rocks, during their consolidation, we must admit in the case of the thin
alternate layers of obsidian and pumice described by Humboldt, and of the
small, flattened, crenulated air-cells in the laminated rocks of Ascension;
for on no other principle can we conceive why the confined vapours should
through their expansion form air-cells or fibres in separate, parallel
planes, instead of irregularly throughout the mass. In Mr. Stokes'
collection, I have seen a beautiful example of this structure, in a
specimen of obsidian from Mexico, which is shaded and zoned, like the
finest agate, with numerous, straight, parallel layers, more or less opaque
and white, or almost perfectly glassy; the degree of opacity and glassiness
depending on the number of microscopically minute, flattened air-cells; in
this case, it is scarcely possible to doubt but that the mass, to which the
fragment belonged, must have been subjected to some, probably prolonged,
action, causing the tension slightly to vary in the successive planes.

Several causes appear capable of producing zones of different tension, in
masses semi-liquified by heat. In a fragment of devitrified glass, I have
observed layers of sphaerulites which appeared, from the manner in which
they were abruptly bent, to have been produced by the simple contraction of
the mass in the vessel, in which it cooled. In certain dikes on Mount Etna,
described by M. Elie de Beaumont ("Mem. pour servir" etc. tome 4 page
131.), as bordered by alternating bands of scoriaceous and compact rock,
one is led to suppose that the stretching movement of the surrounding
strata, which originally produced the fissures, continued whilst the
injected rock remained fluid. Guided, however, by Professor Forbes'
("Edinburgh New Phil. Journal" 1842 page 350.) clear description of the
zoned structure of glacier-ice, far the most probable explanation of the
laminated structure of these feldspathic rocks appears to be, that they
have been stretched whilst slowly flowing onwards in a pasty condition (I
presume that this is nearly the same explanation which Mr. Scrope had in
his mind, when he speaks ("Geolog. Transact." volume 2 second series page
228) of the ribboned structure of his trachytic rocks, having arisen, from
"a linear extension of the mass, while in a state of imperfect liquidity,
coupled with a concretionary process."), in precisely the same manner as
Professor Forbes believes, that the ice of moving glaciers is stretched and
fissured. In both cases, the zones may be compared to those in the finest
agates; in both, they extend in the direction in which the mass has flowed,
and those exposed on the surface are generally vertical: in the ice, the
porous laminae are rendered distinct by the subsequent congelation of
infiltrated water, in the stony feldspathic lavas, by subsequent
crystalline and concretionary action. The fragment of glassy obsidian in
Mr. Stokes' collection, which is zoned with minute air-cells must
strikingly resemble, judging from Professor Forbes' descriptions, a
fragment of the zoned ice; and if the rate of cooling and nature of the
mass had been favourable to its crystallisation or to concretionary action,
we should here have had the finest parallel zones of different composition
and texture. In glaciers, the lines of porous ice and of minute crevices
seem to be due to an incipient stretching, caused by the central parts of
the frozen stream moving faster than the sides and bottom, which are
retarded by friction: hence in glaciers of certain forms and towards the
lower end of most glaciers, the zones become horizontal. May we venture to
suppose that in the feldspathic lavas with horizontal laminae, we see an
analogous case? All geologists, who have examined trachytic regions, have
come to the conclusion, that the lavas of this series have possessed an
exceedingly imperfect fluidity; and as it is evident that only matter thus
characterised would be subject to become fissured and to be formed into
zones of different tensions, in the manner here supposed, we probably see
the reason why augitic lavas, which appear generally to have possessed a
high degree of fluidity, are not, like the feldspathic lavas, divided into
laminae of different composition and texture. (Basaltic lavas, and many
other rocks, are not unfrequently divided into thick laminae or plates, of
the same composition, which are either straight or curved; these being
crossed by vertical lines of fissure, sometimes become united into columns.
This structure seems related, in its origin, to that by which many rocks,
both igneous and sedimentary, become traversed by parallel systems of
fissures.) Moreover, in the augitic series, there never appears to be any
tendency to concretionary action, which we have seen plays an important
part in the lamination of rocks, of the trachytic series, or at least in
rendering that structure apparent.

Whatever may be thought of the explanation here advanced of the laminated
structure of the rocks of the trachytic series, I venture to call the
attention of geologists to the simple fact, that in a body of rock at
Ascension, undoubtedly of volcanic origin, layers often of extreme tenuity,
quite straight, and parallel to each other, have been produced;--some
composed of distinct crystals of quartz and diopside, mingled with
amorphous augitic specks and granular feldspar,--others entirely composed
of these black augitic specks, with granules of oxide of iron,--and lastly,
others formed of crystalline feldspar, in a more or less perfect state of
purity, together with numerous crystals of feldspar, placed lengthways. At
this island, there is reason to believe, and in some analogous cases, it is
certainly known, that the laminae have originally been formed with their
present high inclination. Facts of this nature are manifestly of
importance, with relation to the structural origin of that grand series of
plutonic rocks, which like the volcanic have undergone the action of heat,
and which consist of alternate layers of quartz, feldspar, mica and other


Lavas of the feldspathic, basaltic, and submarine series.
Section of Flagstaff Hill and of the Barn.
Turk's Cap and Prosperous Bays.
Basaltic ring.
Central crateriform ridge, with an internal ledge and a parapet.
Cones of phonolite.
Superficial beds of calcareous sandstone.
Extinct land-shells.
Beds of detritus.
Elevation of the land.
Craters of elevation.

The whole island is of volcanic origin; its circumference, according to
Beatson, is about twenty-eight miles. (Governor Beatson "Account of St.
Helena.") The central and largest part consists of rocks of a feldspathic
nature, generally decomposed to an extraordinary degree; and when in this
state, presenting a singular assemblage of alternating, red, purple, brown,
yellow, and white, soft, argillaceous beds. From the shortness of our
visit, I did not examine these beds with care; some of them, especially
those of the white, yellow, and brown shades, originally existed as streams
of lava, but the greater number were probably ejected in the form of
scoriae and ashes: other beds of a purple tint, porphyritic with crystal-
shaped patches of a white, soft substance, which are now unctuous, and
yield, like wax, a polished streak to the nail, seem once to have existed
as solid claystone-porphyries: the red argillaceous beds generally have a
brecciated structure, and no doubt have been formed by the decomposition of
scoriae. Several extensive streams, however, belonging to this series,
retain their stony character; these are either of a blackish-green colour,
with minute acicular crystals of feldspar, or of a very pale tint, and
almost composed of minute, often scaly, crystals of feldspar, abounding
with microscopical black specks; they are generally compact and laminated;
others, however, of similar composition, are cellular and somewhat
decomposed. None of these rocks contain large crystals of feldspar, or have
the harsh fracture peculiar to trachyte. These feldspathic lavas and tuffs
are the uppermost or those last erupted; innumerable dikes, however, and
great masses of molten rock, have subsequently been injected into them.
They converge, as they rise, towards the central curved ridge, of which one
point attains the elevation of 2,700 feet. This ridge is the highest land
in the island; and it once formed the northern rim of a great crater,
whence the lavas of this series flowed: from its ruined condition, from the
southern half having been removed, and from the violent dislocation which
the whole island has undergone, its structure is rendered very obscure.


The margin of the island is formed by a rude circle of great, black,
stratified, ramparts of basalt, dipping seaward, and worn into cliffs,
which are often nearly perpendicular, and vary in height from a few hundred
feet to two thousand. This circle, or rather horse-shoe shaped ring, is
open to the south, and is breached by several other wide spaces. Its rim or
summit generally projects little above the level of the adjoining inland
country; and the more recent feldspathic lavas, sloping down from the
central heights, generally abut against and overlap its inner margin; on
the north-western side of the island, however, they appear (judging from a
distance) to have flowed over and concealed portions of it. In some parts,
where the basaltic ring has been breached, and the black ramparts stand
detached, the feldspathic lavas have passed between them, and now overhang
the sea-coast in lofty cliffs. The basaltic rocks are of a black colour and
thinly stratified; they are generally highly vesicular, but occasionally
compact; some of them contain numerous crystals of glassy feldspar and
octahedrons of titaniferous iron; others abound with crystals of augite and
grains of olivine. The vesicles are frequently lined with minute crystals
(of chabasie?) and even become amygdaloidal with them. The streams are
separated from each other by cindery matter, or by a bright red, friable,
saliferous tuff, which is marked by successive lines like those of aqueous
deposition; and sometimes it has an obscure, concretionary structure. The
rocks of this basaltic series occur nowhere except near the coast. In most
volcanic districts the trachytic lavas are of anterior origin to the
basaltic; but here we see, that a great pile of rock, closely related in
composition to the trachytic family, has been erupted subsequently to the
basaltic strata: the number, however, of dikes, abounding with large
crystals of augite, with which the feldspathic lavas have been injected,
shows perhaps some tendency to a return to the more usual order of


The lavas of this basal series lie immediately beneath both the basaltic
and feldspathic rocks. According to Mr. Seale, they may be seen at
intervals on the sea-beach round the entire island. ("Geognosy of the
Island of St. Helena." Mr. Seale has constructed a gigantic model of St.
Helena, well worth visiting, which is now deposited at Addiscombe College,
in Surrey.) In the sections which I examined, their nature varied much;
some of the strata abound with crystals of augite; others are of a brown
colour, either laminated or in a rubbly condition; and many parts are
highly amygdaloidal with calcareous matter. The successive sheets are
either closely united together, or are separated from each other by beds of
scoriaceous rock and of laminated tuff, frequently containing well-rounded
fragments. The interstices of these beds are filled with gypsum and salt;
the gypsum also sometimes occurring in thin layers. From the large quantity
of these two substances, from the presence of rounded pebbles in the tuffs,
and from the abundant amygdaloids, I cannot doubt that these basal volcanic
strata flowed beneath the sea. This remark ought perhaps to be extended to
a part of the superincumbent basaltic rocks; but on this point, I was not
able to obtain clear evidence. The strata of the basal series, whenever I
examined them, were intersected by an extraordinary number of dikes.


(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East
(right)) Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high.

The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata; the dikes are
shaded transversely.)

I will now describe some of the more remarkable sections, and will commence
with these two hills, which form the principal external feature on the
north-eastern side of the island. The square, angular outline, and black
colour of the Barn, at once show that it belongs to the basaltic series;
whilst the smooth, conical figure, and the varied bright tints of Flagstaff
Hill, render it equally clear, that it is composed of the softened,
feldspathic rocks. These two lofty hills are connected (as is shown in
Figure 8) by a sharp ridge, which is composed of the rubbly lavas of the
basal series. The strata of this ridge dip westward, the inclination
becoming less and less towards the Flagstaff; and the upper feldspathic
strata of this hill can be seen, though with some difficulty, to dip
conformably to the W.S.W. Close to the Barn, the strata of the ridge are
nearly vertical, but are much obscured by innumerable dikes; under this
hill, they probably change from being vertical into being inclined into an
opposite direction; for the upper or basaltic strata, which are about eight
hundred or one thousand feet in thickness, are inclined north-eastward, at
an angle between thirty and forty degrees.

This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by
dikes, many of which preserve a remarkable parallelism in a N.N.W. and
S.S.E. direction. The dikes chiefly consist of a rock, porphyritic with
large crystals of augite; others are formed of a fine-grained and brown-
coloured trap. Most of these dikes are coated by a glossy layer, from one
to two-tenths of an inch in thickness, which, unlike true pitchstone, fuses
into a black enamel; this layer is evidently analogous to the glossy
superficial coating of many lava streams. (This circumstance has been
observed (Lyell "Principles of Geology" volume 4 chapter 10 page 9) in the
dikes of the Atrio del Cavallo, but apparently it is not of very common
occurrence. Sir G. Mackenzie, however, states (page 372 "Travels in
Iceland") that all the veins in Iceland have a "black vitreous coating on
their sides." Captain Carmichael, speaking of the dikes in Tristan
d'Acunha, a volcanic island in the Southern Atlantic, says ("Linnaean
Transactions" volume 12 page 485) that their sides, "where they come in
contact with the rocks, are invariably in a semi-vitrified state.") The
dikes can often be followed for great lengths both horizontally and
vertically, and they seem to preserve a nearly uniform thickness ("Geognosy
of the Island of St. Helena" plate 5.): Mr. Seale states, that one near the
Barn, in a height of 1,260 feet, decreases in width only four inches,--from
nine feet at the bottom, to eight feet and eight inches at the top. On the
ridge, the dikes appear to have been guided in their course, to a
considerable degree, by the alternating soft and hard strata: they are
often firmly united to the harder strata, and they preserve their
parallelism for such great lengths, that in very many instances it was
impossible to conjecture, which of the beds were dikes, and which streams
of lava. The dikes, though so numerous on this ridge, are even more
numerous in the valleys a little south of it, and to a degree I never saw
equalled anywhere else: in these valleys they extend in less regular lines,
covering the ground with a network, like a spider's web, and with some
parts of the surface even appearing to consist wholly of dikes, interlaced
by other dikes.

From the complexity produced by the dikes, from the high inclination and
anticlinal dip of the strata of the basal series, which are overlaid, at
the opposite ends of the short ridge, by two great masses of different ages
and of different composition, I am not surprised that this singular section
has been misunderstood. It has even been supposed to form part of a crater;
but so far is this from having been the case, that the summit of Flagstaff
Hill once formed the lower extremity of a sheet of lava and ashes, which
were erupted from the central, crateriform ridge. Judging from the slope of
the contemporaneous streams in an adjoining and undisturbed part of the
island, the strata of the Flagstaff Hill must have been upturned at least
twelve hundred feet, and probably much more, for the great truncated dikes
on its summit show that it has been largely denuded. The summit of this
hill now nearly equals in height the crateriform ridge; and before having
been denuded, it was probably higher than this ridge, from which it is
separated by a broad and much lower tract of country; we here, therefore,
see that the lower extremities of a set of lava-streams have been tilted up
to as great a height as, or perhaps greater height than, the crater, down
the flanks of which they originally flowed. I believe that dislocations on
so grand a scale are extremely rare in volcanic districts. (M. Constant
Prevost "Mem. de la Soc. Geolog." tome 2 observes that "les produits
volcaniques n'ont que localement et rarement meme derange le sol, a travers
lequel ils se sont fait jour.") The formation of such numbers of dikes in
this part of the island shows that the surface must here have been
stretched to a quite extraordinary degree: this stretching, on the ridge
between Flagstaff and Barn Hills, probably took place subsequently (though
perhaps immediately so) to the strata being tilted; for had the strata at
that time extended horizontally, they would in all probability have been
fissured and injected transversely, instead of in the planes of their
stratification. Although the space between the Barn and Flagstaff Hill
presents a distinct anticlinal line extending north and south, and though
most of the dikes range with much regularity in the same line,
nevertheless, at only a mile due south of the ridge the strata lie
undisturbed. Hence the disturbing force seems to have acted under a point,
rather than along a line. The manner in which it has acted, is probably
explained by the structure of Little Stony-top, a mountain 2,000 feet high,
situated a few miles southward of the Barn; we there see, even from a
distance, a dark-coloured, sharp, wedge of compact columnar rock, with the
bright-coloured feldspathic strata, sloping away on each side from its
uncovered apex. This wedge, from which it derives its name of Stony-top,
consists of a body of rock, which has been injected whilst liquified into
the overlying strata; and if we may suppose that a similar body of rock
lies injected, beneath the ridge connecting the Barn and Flagstaff, the
structure there exhibited would be explained.


(right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill to The

The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata.)

Prosperous Hill is a great, black, precipitous mountain, situated two miles
and a half south of the Barn, and composed, like it, of basaltic strata.
These rest, in one part, on the brown-coloured, porphyritic beds of the
basal series, and in another part, on a fissured mass of highly scoriaceous
and amygdaloidal rock, which seems to have formed a small point of eruption
beneath the sea, contemporaneously with the basal series. Prosperous Hill,
like the Barn, is traversed by many dikes, of which the greater number
range north and south, and its strata dip, at an angle of about 20 degrees,
rather obliquely from the island towards the sea. The space between
Prosperous Hill and the Barn, as represented in Figure 9, consists of lofty
cliffs, composed of the lavas of the upper or feldspathic series, which
rest, though unconformably, on the basal submarine strata, as we have seen
that they do at Flagstaff Hill. Differently, however, from in that hill,
these upper strata are nearly horizontal, gently rising towards the
interior of the island; and they are composed of greenish-black, or more
commonly, pale brown, compact lavas, instead of softened and highly
coloured matter. These brown-coloured, compact lavas, consist almost
entirely of small glimmering scales, or of minute acicular crystals, of
feldspar, placed close by the side of each other, and abounding with minute
black specks, apparently of hornblende. The basaltic strata of Prosperous
Hill project only a little above the level of the gently-sloping,
feldspathic streams, which wind round and abut against their upturned
edges. The inclination of the basaltic strata seems to be too great to have
been caused by their having flowed down a slope, and they must have been
tilted into their present position before the eruption of the feldspathic


Proceeding round the Island, the lavas of the upper series, southward of
Prosperous Hill, overhang the sea in lofty precipices. Further on, the
headland, called Great Stony-top, is composed, as I believe, of basalt; as
is Long Range Point, on the inland side of which the coloured beds abut. On
the southern side of the island, we see the basaltic strata of the South
Barn, dipping obliquely seaward at a considerable angle; this headland,
also, stands a little above the level of the more modern, feldspathic
lavas. Further on, a large space of coast, on each side of Sandy Bay, has
been much denuded, and there seems to be left only the basal wreck of the
great, central crater. The basaltic strata reappear, with their seaward
dip, at the foot of the hill, called Man-and-Horse; and thence they are
continued along the whole north-western coast to Sugar-Loaf Hill, situated
near to the Flagstaff; and they everywhere have the same seaward
inclination, and rest, in some parts at least, on the lavas of the basal
series. We thus see that the circumference of the island is formed by a
much-broken ring, or rather, a horse-shoe, of basalt, open to the south,
and interrupted on the eastern side by many wide breaches. The breadth of
this marginal fringe on the north-western side, where alone it is at all
perfect, appears to vary from a mile to a mile and a half. The basaltic
strata, as well as those of the subjacent basal series, dip, with a
moderate inclination, where they have not been subsequently disturbed,
towards the sea. The more broken state of the basaltic ring round the
eastern half, compared with the western half of the island, is evidently
due to the much greater denuding power of the waves on the eastern or
windward side, as is shown by the greater height of the cliffs on that
side, than to leeward. Whether the margin of basalt was breached, before or
after the eruption of the lavas of the upper series, is doubtful; but as
separate portions of the basaltic ring appear to have been tilted before
that event, and from other reasons, it is more probable, that some at least
of the breaches were first formed. Reconstructing in imagination, as far as
is possible, the ring of basalt, the internal space or hollow, which has
since been filled up with the matter erupted from the great central crater,
appears to have been of an oval figure, eight or nine miles in length by
about four miles in breadth, and with its axis directed in a N.E. and S W.
line, coincident with the present longest axis of the island.


This ridge consists, as before remarked, of grey feldspathic lavas, and of
red, brecciated, argillaceous tuffs, like the beds of the upper coloured
series. The grey lavas contain numerous, minute, black, easily fusible
specks; and but very few large crystals of feldspar. They are generally
much softened; with the exception of this character, and of being in many
parts highly cellular, they are quite similar to those great sheets of lava
which overhang the coast at Prosperous Bay. Considerable intervals of time
appear to have elapsed, judging from the marks of denudation, between the
formation of the successive beds, of which this ridge is composed. On the
steep northern slope, I observed in several sections a much worn undulating
surface of red tuff, covered by grey, decomposed, feldspathic lavas, with
only a thin earthy layer interposed between them. In an adjoining part, I
noticed a trap-dike, four feet wide, cut off and covered up by the
feldspathic lava, as is represented in Figure 9. The ridge ends on the
eastern side in a hook, which is not represented clearly enough in any map
which I have seen; towards the western end, it gradually slopes down and
divides into several subordinate ridges. The best defined portion between
Diana's Peak and Nest Lodge, which supports the highest pinnacles in the
island varying from 2,000 to 2,700 feet, is rather less than three miles
long in a straight line. Throughout this space the ridge has a uniform
appearance and structure; its curvature resembles that of the coast-line of
a great bay, being made up of many smaller curves, all open to the south.
The northern and outer side is supported by narrow ridges or buttresses,
which slope down to the adjoining country. The inside is much steeper, and
is almost precipitous; it is formed of the basset edges of the strata,
which gently decline outwards. Along some parts of the inner side, a little
way beneath the summit, a flat ledge extends, which imitates in outline the
smaller curvatures of the crest. Ledges of this kind occur not unfrequently
within volcanic craters, and their formation seems to be due to the sinking
down of a level sheet of hardened lava, the edges of which remain (like the
ice round a pool, from which the water has been drained) adhering to the
sides. (A most remarkable instance of this structure is described in Ellis
"Polynesian Researches" second edition where an admirable drawing is given
of the successive ledges or terraces, on the borders of the immense crater
at Hawaii, in the Sandwich Islands.)

(FIGURE 10. DIKE. (Section showing layers 1, 2 and 3 from top to bottom.)

1. Grey feldspathic lava.

2. A layer, one inch in thickness, of a reddish earthy matter.

3. Brecciated, red, argillaceous tuff.)

In some parts, the ridge is surmounted by a wall or parapet, perpendicular
on both sides. Near Diana's Peak this wall is extremely narrow. At the
Galapagos Archipelago I observed parapets, having a quite similar structure
and appearance, surmounting several of the craters; one, which I more
particularly examined, was composed of glossy, red scoriae firmly cemented
together; being externally perpendicular, and extending round nearly the
whole circumference of the crater, it rendered it almost inaccessible. The
Peak of Teneriffe and Cotopaxi, according to Humboldt, are similarly
constructed; he states that "at their summits a circular wall surrounds the
crater, which wall, at a distance, has the appearance of a small cylinder
placed on a truncated cone. ("Personal Narrative" volume 1 page 171.) On
Cotopaxi this peculiar structure is visible to the naked eye at more than
two thousand toises' distance; and no person has ever reached its crater.
(Humboldt "Picturesque Atlas" folio plate 10.) On the Peak of Teneriffe,
the parapet is so high, that it would be impossible to reach the caldera,
if on the eastern side there did not exist a breach." The origin of these
circular parapets is probably due to the heat or vapours from the crater,
penetrating and hardening the sides to a nearly equal depth, and afterwards
to the mountain being slowly acted on by the weather, which would leave the
hardened part, projecting in the form of a cylinder or circular parapet.

From the points of structure in the central ridge, now enumerated,--namely,
from the convergence towards it of the beds of the upper series,--from the
lavas there becoming highly cellular,--from the flat ledge, extending along
its inner and precipitous side, like that within some still active
craters,--from the parapet-like wall on its summit,--and lastly, from its
peculiar curvature, unlike that of any common line of elevation, I cannot
doubt that this curved ridge forms the last remnant of a great crater. In
endeavouring, however, to trace its former outline, one is soon baffled;
its western extremity gradually slopes down, and, branching into other
ridges, extends to the sea-coast; the eastern end is more curved, but it is
only a little better defined. Some appearances lead me to suppose that the
southern wall of the crater joined the present ridge near Nest Lodge; in
this case the crater must have been nearly three miles long, and about a
mile and a half in breadth. Had the denudation of the ridge and the
decomposition of its constituent rocks proceeded a few steps further, and
had this ridge, like several other parts of the island, been broken up by
great dikes and masses of injected matter, we should in vain have
endeavoured to discover its true nature. Even now we have seen that at
Flagstaff Hill the lower extremity and most distant portion of one sheet of
the erupted matter has been upheaved to as great a height as the crater
down which it flowed, and probably even to a greater height. It is
interesting thus to trace the steps by which the structure of a volcanic
district becomes obscured, and finally obliterated: so near to this last
stage is St. Helena, that I believe no one has hitherto suspected that the
central ridge or axis of the island is the last wreck of the crater, whence
the most modern volcanic streams were poured forth.

The great hollow space or valley southward of the central curved ridge,
across which the half of the crater must once have extended, is formed of
bare, water-worn hillocks and ridges of red, yellow, and brown rocks,
mingled together in chaos-like confusion, interlaced by dikes, and without
any regular stratification. The chief part consists of red decomposing
scoriae, associated with various kinds of tuff and yellow argillaceous
beds, full of broken crystals, those of augite being particularly large.
Here and there masses of highly cellular and amygdaloidal lavas protrude.
From one of the ridges in the midst of the valley, a conical precipitous
hill, called Lot, boldly stands up, and forms a most singular and
conspicuous object. It is composed of phonolite, divided in one part into
great curved laminae, in another, into angular concretionary balls, and in
a third part into outwardly radiating columns. At its base the strata of
lava, tuff, and scoriae, dip away on all sides (Abich in his "Views of
Vesuvius" plate 6 has shown the manner in which beds, under nearly similar
circumstances, are tilted up. The upper beds are more turned up than the
lower; and he accounts for this, by showing that the lava insinuates itself
horizontally between the lower beds.); the uncovered portion is 197 feet in
height (This height is given by Mr. Seale in his Geognosy of the island.
The height of the summit above the level of the sea is said to be 1,444
feet.), and its horizontal section gives an oval figure. The phonolite is
of a greenish-grey colour, and is full of minute acicular crystals of
feldspar; in most parts it has a conchoidal fracture, and is sonorous, yet
it is crenulated with minute air-cavities. In a S.W. direction from Lot,
there are some other remarkable columnar pinnacles, but of a less regular
shape, namely, Lot's Wife, and the Asses' Ears, composed of allied kinds of
rock. From their flattened shape, and their relative position to each
other, they are evidently connected on the same line of fissure. It is,
moreover, remarkable that this same N.E. and S.W. line, joining Lot and
Lot's Wife, if prolonged would intersect Flagstaff Hill, which, as before
stated, is crossed by numerous dikes running in this direction, and which
has a disturbed structure, rendering it probable that a great body of once
fluid rock lies injected beneath it.

In this same great valley there are several other conical masses of
injected rock (one, I observed, was composed of compact greenstone), some
of which are not connected, as far as is apparent, with any line of dike;
whilst others are obviously thus connected. Of these dikes, three or four
great lines stretch across the valley in a N.E. and S.W. direction,
parallel to that one connecting the Asses' Ears, Lot's Wife, and probably
Lot. The number of these masses of injected rock is a remarkable feature in
the geology of St. Helena. Besides those just mentioned, and the
hypothetical one beneath Flagstaff Hill, there is Little Stony-top and
others, as I have reason to believe, at the Man-and-Horse, and at High
Hill. Most of these masses, if not all of them, have been injected
subsequently to the last volcanic eruptions from the central crater. The
formation of conical bosses of rock on lines of fissure, the walls of which
are in most cases parallel, may probably be attributed to inequalities in
the tension, causing small transverse fissures, and at these points of
intersection the edges of the strata would naturally yield, and be easily
turned upwards. Finally, I may remark, that hills of phonolite everywhere
are apt to assume singular and even grotesque shapes, like that of Lot
(D'Aubuisson in his "Traite de Geognosie" tome 2 page 540 particularly
remarks that this is the case.): the peak at Fernando Noronha offers an
instance; at St. Jago, however, the cones of phonolite, though tapering,
have a regular form. Supposing, as seems probable, that all such hillocks
or obelisks have originally been injected, whilst liquified, into a mould
formed by yielding strata, as certainly has been the case with Lot, how are
we to account for the frequent abruptness and singularity of their
outlines, compared with similarly injected masses of greenstone and basalt?
Can it be due to a less perfect degree of fluidity, which is generally
supposed to be characteristic of the allied trachytic lavas?


Soft calcareous sandstone occurs in extensive, though thin, superficial
beds, both on the northern and southern shores of the island. It consists
of very minute, equal-sized, rounded particles of shells, and other organic
bodies, which partially retain their yellow, brown, and pink colours, and
occasionally, though very rarely, present an obscure trace of their
original external forms. I in vain endeavoured to find a single unrolled
fragment of a shell. The colour of the particles is the most obvious
character by which their origin can be recognised, the tints being affected
(and an odour produced) by a moderate heat, in the same manner as in fresh
shells. The particles are cemented together, and are mingled with some
earthy matter: the purest masses, according to Beatson, contain 70 per cent
of carbonate of lime. The beds, varying in thickness from two or three feet
to fifteen feet, coat the surface of the ground; they generally lie on that
side of the valley which is protected from the wind, and they occur at the
height of several hundred feet above the level of the sea. Their position
is the same which sand, if now drifted by the trade-wind, would occupy; and
no doubt they thus originated, which explains the equal size and minuteness
of the particles, and likewise the entire absence of whole shells, or even
of moderately-sized fragments. It is remarkable that at the present day
there are no shelly beaches on any part of the coast, whence calcareous
dust could be drifted and winnowed; we must, therefore, look back to a
former period when before the land was worn into the present great
precipices, a shelving coast, like that of Ascension, was favourable to the
accumulation of shelly detritus. Some of the beds of this limestone are
between six hundred and seven hundred feet above the sea; but part of this
height may possibly be due to an elevation of the land, subsequent to the
accumulation of the calcareous sand.

The percolation of rain-water has consolidated parts of these beds into a
solid rock, and has formed masses of dark brown, stalagmitic limestone. At
the Sugar-Loaf quarry, fragments of rock on the adjoining slopes have been
thickly coated by successive fine layers of calcareous matter. (In the
earthy detritus on several parts of this hill, irregular masses of very
impure, crystallised sulphate of lime occur. As this substance is now being
abundantly deposited by the surf at Ascension, it is possible that these
masses may thus have originated; but if so, it must have been at a period
when the land stood at a much lower level. This earthy selenite is now
found at a height of between six hundred and seven hundred feet.) It is
singular, that many of these pebbles have their entire surfaces coated,
without any point of contact having been left uncovered; hence, these
pebbles must have been lifted up by the slow deposition between them of the
successive films of carbonate of lime. Masses of white, finely oolitic rock
are attached to the outside of some of these coated pebbles. Von Buch has
described a compact limestone at Lanzarote, which seems perfectly to
resemble the stalagmitic deposition just mentioned: it coats pebbles, and
in parts is finely oolitic: it forms a far-extended layer, from one inch to
two or three feet in thickness, and it occurs at the height of 800 feet
above the sea, but only on that side of the island exposed to the violent
north-western winds. Von Buch remarks, that it is not found in hollows, but
only on the unbroken and inclined surfaces of the mountain. ("Description
des Isles Canaries" page 293.) He believes, that it has been deposited by
the spray which is borne over the whole island by these violent winds. It
appears, however, to me much more probable that it has been formed, as at
St. Helena, by the percolation of water through finely comminuted shells:
for when sand is blown on a much-exposed coast, it always tends to
accumulate on broad, even surfaces, which offer a uniform resistance to the
winds. At the neighbouring island, moreover, of Feurteventura, there is an
earthy limestone, which, according to Von Buch, is quite similar to
specimens which he has seen from St. Helena, and which he believes to have
been formed by the drifting of shelly detritus. (Idem pages 314 and 374.)

The upper beds of the limestone, at the above-mentioned quarry on the
Sugar-Loaf Hill, are softer, finer-grained and less pure, than the lower
beds. They abound with fragments of land-shells, and with some perfect
ones; they contain, also, the bones of birds, and the large eggs,
apparently of water-fowl. (Colonel Wilkes, in a catalogue presented with
some specimens to the Geological Society, states that as many as ten eggs
were found by one person. Dr. Buckland has remarked ("Geolog. Trans."
volume 5 page 474) on these eggs.) It is probable that these upper beds
remained long in an unconsolidated form, during which time, these
terrestrial productions were embedded. Mr. G.R. Sowerby has kindly examined
three species of land-shells, which I procured from this bed, and has
described them in detail. One of them is a Succinea, identical with a
species now living abundantly on the island; the two others, namely,
Cochlogena fossilis and Helix biplicata, are not known in a recent state:
the latter species was also found in another and different locality,
associated with a species of Cochlogena which is undoubtedly extinct.


Land-shells, all of which appear to be species now extinct, occur embedded
in earth, in several parts of the island. The greater number have been
found at a considerable height on Flagstaff Hill. On the N.W. side of this
hill, a rain-channel exposes a section of about twenty feet in thickness,
of which the upper part consists of black vegetable mould, evidently washed
down from the heights above, and the lower part of less black earth,
abounding with young and old shells, and with their fragments: part of this
earth is slightly consolidated by calcareous matter, apparently due to the
partial decomposition of some of the shells. Mr. Seale, an intelligent
resident, who first called attention to these shells, gave me a large
collection from another locality, where the shells appear to have been
embedded in very black earth. Mr. G.R. Sowerby has examined these shells,
and has described them. There are seven species, namely, one Cochlogena,
two species of the genus Cochlicopa, and four of Helix; none of these are
known in a recent state, or have been found in any other country. The
smaller species were picked out of the inside of the large shells of the
Cochlogena aurisvulpina. This last-mentioned species is in many respects a
very singular one; it was classed, even by Lamarck, in a marine genus, and
having thus been mistaken for a sea-shell, and the smaller accompanying
species having been overlooked, the exact localities where it was found
have been measured, and the elevation of this island thus deduced! It is
very remarkable that all the shells of this species found by me in one
spot, form a distinct variety, as described by Mr. Sowerby, from those
procured from another locality by Mr. Seale. As this Cochlogena is a large
and conspicuous shell, I particularly inquired from several intelligent
countrymen whether they had ever seen it alive; they all assured me that
they had not, and they would not even believe that it was a land animal:
Mr. Seale, moreover, who was a collector of shells all his life at St.
Helena, never met with it alive. Possibly some of the smaller species may
turn out to be yet living kinds; but, on the other hand, the two land-
shells which are now living on the island in great numbers, do not occur
embedded, as far as is yet known, with the extinct species. I have shown in
my "Journal" ("Journal of Researches" page 582.), that the extinction of
these land-shells possibly may not be an ancient event; as a great change
took place in the state of the island about one hundred and twenty years
ago, when the old trees died, and were not replaced by young ones, these
being destroyed by the goats and hogs, which had run wild in numbers, from
the year 1502. Mr. Seale states, that on Flagstaff Hill, where we have seen
that the embedded land-shells are especially numerous, traces are
everywhere discoverable, which plainly indicate that it was once thickly
clothed with trees; at present not even a bush grows there. The thick bed
of black vegetable mould which covers the shell-bed, on the flanks of this
hill, was probably washed down from the upper part, as soon as the trees
perished, and the shelter afforded by them was lost.


Seeing that the lavas of the basal series, which are of submarine origin,
are raised above the level of the sea, and at some places to the height of
many hundred feet, I looked out for superficial signs of the elevation of
the land. The bottoms of some of the gorges, which descend to the coast,
are filled up to the depth of about a hundred feet, by rudely divided
layers of sand, muddy clay, and fragmentary masses; in these beds, Mr.
Seale has found the bones of the tropic-bird and of the albatross; the
former now rarely, and the latter never visiting the island. From the
difference between these layers, and the sloping piles of detritus which
rest on them, I suspect that they were deposited, when the gorges stood
beneath the sea. Mr. Seale, moreover, has shown that some of the fissure-
like gorges become, with a concave outline, gradually rather wider at the
bottom than at the top; and this peculiar structure was probably caused by
the wearing action of the sea, when it entered the lower part of these
gorges. (A fissure-like gorge, near Stony-top, is said by Mr. Seale to be
840 feet deep, and only 115 feet in width.) At greater heights, the
evidence of the rise of the land is even less clear: nevertheless, in a
bay-like depression on the table-land behind Prosperous Bay, at the height
of about a thousand feet, there are flat-topped masses of rock, which it is
scarcely conceivable, could have been insulated from the surrounding and
similar strata, by any other agency than the denuding action of a sea-
beach. Much denudation, indeed, has been effected at great elevations,
which it would not be easy to explain by any other means: thus, the flat
summit of the Barn, which is 2,000 feet high, presents, according to Mr.
Seale, a perfect network of truncated dikes; on hills like the Flagstaff,
formed of soft rock, we might suppose that the dikes had been worn down and
cut off by meteoric agency, but we can hardly suppose this possible with
the hard, basaltic strata of the Barn.


The enormous cliffs, in many parts between one and two thousand feet in
height, with which this prison-like island is surrounded, with the
exception of only a few places, where narrow valleys descend to the coast,
is the most striking feature in its scenery. We have seen that portions of
the basaltic ring, two or three miles in length by one or two miles in
breadth, and from one to two thousand feet in height, have been wholly
removed. There are, also, ledges and banks of rock, rising out of
profoundly deep water, and distant from the present coast between three and
four miles, which, according to Mr. Seale, can be traced to the shore, and
are found to be the continuations of certain well-known great dikes. The
swell of the Atlantic Ocean has obviously been the active power in forming
these cliffs; and it is interesting to observe that the lesser, though
still great, height of the cliffs on the leeward and partially protected
side of the island (extending from the Sugar-Loaf Hill to South West
Point), corresponds with the lesser degree of exposure. When reflecting on
the comparatively low coasts of many volcanic islands, which also stand
exposed in the open ocean, and are apparently of considerable antiquity,
the mind recoils from an attempt to grasp the number of centuries of
exposure, necessary to have ground into mud and to have dispersed the
enormous cubic mass of hard rock which has been pared off the circumference
of this island. The contrast in the superficial state of St. Helena,
compared with the nearest island, namely, Ascension, is very striking. At
Ascension, the surfaces of the lava-streams are glossy, as if just poured
forth, their boundaries are well defined, and they can often be traced to
perfect craters, whence they were erupted; in the course of many long
walks, I did not observe a single dike; and the coast round nearly the
entire circumference is low, and has been eaten back (though too much
stress must not be placed on this fact, as the island may have been
subsiding) into a little wall only from ten to thirty feet high. Yet during
the 340 years, since Ascension has been known, not even the feeblest signs
of volcanic action have been recorded. (In the "Nautical Magazine" for 1835
page 642, and for 1838 page 361, and in the "Comptes Rendus" April 1838,
accounts are given of a series of volcanic phenomena--earthquakes--troubled
water--floating scoriae and columns of smoke--which have been observed at
intervals since the middle of the last century, in a space of open sea
between longitudes 20 degrees and 22 degrees west, about half a degree
south of the equator. These facts seem to show, that an island or an
archipelago is in process of formation in the middle of the Atlantic: a
line joining St. Helena and Ascension, prolonged, intersects this slowly
nascent focus of volcanic action.) On the other hand, at St. Helena, the
course of no one stream of lava can be traced, either by the state of its
boundaries or of its superficies; the mere wreck of one great crater is
left; not the valleys only, but the surfaces of some of the highest hills,
are interlaced by worn-down dikes, and, in many places, the denuded summits
of great cones of injected rock stand exposed and naked; lastly, as we have
seen, the entire circuit of the island has been deeply worn back into the
grandest precipices.


There is much resemblance in structure and in geological history between
St. Helena, St. Jago, and Mauritius. All three islands are bounded (at
least in the parts which I was able to examine) by a ring of basaltic
mountains, now much broken, but evidently once continuous. These mountains
have, or apparently once had, their escarpments steep towards the interior
of the island, and their strata dip outwards. I was able to ascertain, only
in a few cases, the inclination of the beds; nor was this easy, for the
stratification was generally obscure, except when viewed from a distance. I
feel, however, little doubt that, according to the researches of M. Elie de
Beaumont, their average inclination is greater than that which they could
have acquired, considering their thickness and compactness, by flowing down
a sloping surface. At St. Helena, and at St. Jago, the basaltic strata rest
on older and probably submarine beds of different composition. At all three
islands, deluges of more recent lavas have flowed from the centre of the
island, towards and between the basaltic mountains; and at St. Helena the
central platform has been filled up by them. All three islands have been
raised in mass. At Mauritius the sea, within a late geological period, must
have reached to the foot of the basaltic mountains, as it now does at St.
Helena; and at St. Jago it is cutting back the intermediate plain towards
them. In these three islands, but especially at St. Jago and at Mauritius,
when, standing on the summit of one of the old basaltic mountains, one
looks in vain towards the centre of the island,--the point towards which
the strata beneath one's feet, and of the mountains on each side, rudely
converge,--for a source whence these strata could have been erupted; but
one sees only a vast hollow platform stretched beneath, or piles of matter
of more recent origin.

These basaltic mountains come, I presume, into the class of Craters of
elevation: it is immaterial whether the rings were ever completely formed,
for the portions which now exist have so uniform a structure, that, if they
do not form fragments of true craters, they cannot be classed with ordinary
lines of elevation. With respect to their origin, after having read the
works of Mr. Lyell ("Principles of Geology" fifth edition volume 2 page
171.), and of MM. C. Prevost and Virlet, I cannot believe that the great
central hollows have been formed by a simple dome-shaped elevation, and the
consequent arching of the strata. On the other hand, I have very great
difficulty in admitting that these basaltic mountains are merely the basal
fragments of great volcanoes, of which the summits have either been blown
off, or more probably swallowed up by subsidence. These rings are, in some
instances, so immense, as at St. Jago and at Mauritius, and their
occurrence is so frequent, that I can hardly persuade myself to adopt this
explanation. Moreover, I suspect that the following circumstances, from
their frequent concurrence, are someway connected together,--a connection
not implied in either of the above views: namely, first, the broken state
of the ring; showing that the now detached portions have been exposed to
great denudation, and in some cases, perhaps, rendering it probable that
the ring never was entire; secondly, the great amount of matter erupted
from the central area after or during the formation of the ring; and
thirdly, the elevation of the district in mass. As far as relates to the
inclination of the strata being greater than that which the basal fragments
of ordinary volcanoes would naturally possess, I can readily believe that
this inclination might have been slowly acquired by that amount of
elevation, of which, according to M. Elie de Beaumont, the numerous
upfilled fissures or dikes are the evidence and the measure,--a view
equally novel and important, which we owe to the researches of that
geologist on Mount Etna.

A conjecture, including the above circumstances, occurred to me, when,--
with my mind fully convinced, from the phenomena of 1835 in South America,
that the forces which eject matter from volcanic orifices and raise
continents in mass are identical,--I viewed that part of the coast of St.
Jago, where the horizontally upraised, calcareous stratum dips into the
sea, directly beneath a cone of subsequently erupted lava. (I have given a
detailed account of these phenomena, in a paper read before the Geological
Society in March 1838. At the instant of time, when an immense area was
convulsed and a large tract elevated, the districts immediately surrounding
several of the great vents in the Cordillera remained quiescent; the
subterranean forces being apparently relieved by the eruptions, which then
recommenced with great violence. An event of somewhat the same kind, but on
an infinitely smaller scale, appears to have taken place, according to
Abich ("Views of Vesuvius" plates 1 and 9), within the great crater of
Vesuvius, where a platform on one side of a fissure was raised in mass
twenty feet, whilst on the other side, a train of small volcanoes burst
forth in eruption.) The conjecture is that, during the slow elevation of a
volcanic district or island, in the centre of which one or more orifices
continue open, and thus relieve the subterranean forces, the borders are
elevated more than the central area; and that the portions thus upraised do
not slope gently into the central, less elevated area, as does the
calcareous stratum under the cone at St. Jago, and as does a large part of
the circumference of Iceland, but that they are separated from it by curved
faults. (It appears, from information communicated to me in the most
obliging manner by M. E. Robert, that the circumferential parts of Iceland,
which are composed of ancient basaltic strata alternating with tuff, dip
inland, thus forming a gigantic saucer. M. Robert found that this was the
case, with a few and quite local exceptions, for a space of coast several
hundred miles in length. I find this statement corroborated, as far as
regards one place, by Mackenzie in his "Travels" page 377, and in another
place by some MS. notes kindly lent me by Dr. Holland. The coast is deeply
indented by creeks, at the head of which the land is generally low. M.
Robert informs me, that the inwardly dipping strata appear to extend as far
as this line, and that their inclination usually corresponds with the slope
of the surface, from the high coast-mountains to the low land at the head
of these creeks. In the section described by Sir G. Mackenzie, the dip is
120. The interior parts of the island chiefly consist, as far as is known,
of recently erupted matter. The great size, however, of Iceland, equalling
the bulkiest part of England, ought perhaps to exclude it from the class of
islands we have been considering; but I cannot avoid suspecting that if the
coast-mountains, instead of gently sloping into the less elevated central
area, had been separated from it by irregularly curved faults, the strata
would have been tilted seaward, and a "Crater of elevation," like that of
St. Jago or that of Mauritius, but of much vaster dimensions, would have
been formed. I will only further remark, that the frequent occurrence of
extensive lakes at the foot of large volcanoes, and the frequent
association of volcanic and fresh-water strata, seem to indicate that the
areas around volcanoes are apt to be depressed beneath the level of the
adjoining country, either from having been less elevated, or from the
effects of subsidence.) We might expect, from what we see along ordinary
faults, that the strata on the upraised side, already dipping outwards from
their original formation as lava-streams, would be tilted from the line of
fault, and thus have their inclination increased. According to this
hypothesis, which I am tempted to extend only to some few cases, it is not
probable that the ring would ever be formed quite perfect; and from the
elevation being slow, the upraised portions would generally be exposed to
much denudation, and hence the ring become broken; we might also expect to
find occasional inequalities in the dip of the upraised masses, as is the
case at St. Jago. By this hypothesis the elevation of the districts in
mass, and the flowing of deluges of lava from the central platforms, are
likewise connected together. On this view the marginal basaltic mountains
of the three foregoing islands might still be considered as forming
"Craters of elevation;" the kind of elevation implied having been slow, and
the central hollow or platform having been formed, not by the arching of
the surface, but simply by that part having been upraised to a less height.


Chatham Island.
Craters composed of a peculiar kind of tuff.
Small basaltic craters, with hollows at their bases.
Albemarle Island; fluid lavas, their composition.
Craters of tuff; inclination of their exterior diverging strata, and
structure of their interior converging strata.
James Island, segment of a small basaltic crater; fluidity and composition
of its lava-streams, and of its ejected fragments.
Concluding remarks on the craters of tuff, and on the breached condition of
their southern sides.
Mineralogical composition of the rocks of the archipelago.
Elevation of the land.
Direction of the fissures of eruption.


Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle, James,
Indefatigable, Barrington, Chatham, Charles and Hood's Islands.)

This archipelago is situated under the equator, at a distance of between
five and six hundred miles from the west coast of South America. It
consists of five principal islands, and of several small ones, which
together are equal in area, but not in extent of land, to Sicily,
conjointly with the Ionian Islands. (I exclude from this measurement, the
small volcanic islands of Culpepper and Wenman, lying seventy miles
northward of the group. Craters were visible on all the islands of the
group, except on Towers Island, which is one of the lowest; this island is,
however, formed of volcanic rocks.) They are all volcanic: on two, craters
have been seen in eruption, and on several of the other islands, streams of
lava have a recent appearance. The larger islands are chiefly composed of
solid rock, and they rise with a tame outline to a height of between one
and four thousand feet. They are sometimes, but not generally, surmounted
by one principal orifice. The craters vary in size from mere spiracles to
huge caldrons several miles in circumference; they are extraordinarily
numerous, so that I should think, if enumerated, they would be found to
exceed two thousand; they are formed either of scoriae and lava, or of a
brown-coloured tuff; and these latter craters are in several respects
remarkable. The whole group was surveyed by the officers of the "Beagle." I
visited myself four of the principal islands, and received specimens from
all the others. Under the head of the different islands I will describe
only that which appears to me deserving of attention.


Towards the eastern end of this island there occur two craters composed of
two kinds of tuff; one kind being friable, like slightly consolidated
ashes; and the other compact, and of a different nature from anything which
I have met with described. This latter substance, where it is best
characterised, is of a yellowish-brown colour, translucent, and with a
lustre somewhat resembling resin; it is brittle, with an angular, rough,
and very irregular fracture, sometimes, however, being slightly granular,
and even obscurely crystalline: it can readily be scratched with a knife,
yet some points are hard enough just to mark common glass; it fuses with
ease into a blackish-green glass. The mass contains numerous broken
crystals of olivine and augite, and small particles of black and brown
scoriae; it is often traversed by thin seams of calcareous matter. It
generally affects a nodular or concretionary structure. In a hand specimen,
this substance would certainly be mistaken for a pale and peculiar variety
of pitchstone; but when seen in mass its stratification, and the numerous
layers of fragments of basalt, both angular and rounded, at once render its
subaqueous origin evident. An examination of a series of specimens shows
that this resin-like substance results from a chemical change on small
particles of pale and dark-coloured scoriaceous rocks; and this change
could be distinctly traced in different stages round the edges of even the
same particle. The position near the coast of all the craters composed of
this kind of tuff or peperino, and their breached condition, renders it
probable that they were all formed when standing immersed in the sea;
considering this circumstance, together with the remarkable absence of
large beds of ashes in the whole archipelago, I think it highly probable
that much the greater part of the tuff has originated from the trituration
of fragments of the grey, basaltic lavas in the mouths of craters standing
in the sea. It may be asked whether the heated water within these craters
has produced this singular change in the small scoriaceous particles and
given to them their translucent, resin-like fracture. Or has the associated
lime played any part in this change? I ask these questions from having
found at St. Jago, in the Cape de Verde Islands, that where a great stream
of molten lava has flowed over a calcareous bottom into the sea, the
outermost film, which in other parts resembles pitchstone, is changed,
apparently by its contact with the carbonate of lime, into a resin-like
substance, precisely like the best characterised specimens of the tuff from
this archipelago. (The concretions containing lime, which I have described
at Ascension, as formed in a bed of ashes, present some degree of
resemblance to this substance, but they have not a resinous fracture. At
St. Helena, also, I found veins of a somewhat similar, compact, but non-
resinous substance, occurring in a bed of pumiceous ashes, apparently free
from calcareous matter: in neither of these cases could heat have acted.)

To return to the two craters: one of them stands at the distance of a
league from the coast, the intervening tract consisting of a calcareous
tuff, apparently of submarine origin. This crater consists of a circle of
hills some of which stand quite detached, but all have a very regular, qua-
qua versal dip, at an inclination of between thirty and forty degrees. The
lower beds, to the thickness of several hundred feet, consist of the resin-
like stone, with embedded fragments of lava. The upper beds, which are
between thirty and forty feet in thickness, are composed of a thinly
stratified, fine-grained, harsh, friable, brown-coloured tuff, or peperino.
(Those geologists who restrict the term of "tuff" to ashes of a white
colour, resulting from the attrition of feldspathic lavas, would call these
brown-coloured strata "peperino.") A central mass without any
stratification, which must formerly have occupied the hollow of the crater,
but is now attached only to a few of the circumferential hills, consists of
a tuff, intermediate in character between that with a resin-like, and that
with an earthy fracture. This mass contains white calcareous matter in
small patches. The second crater (520 feet in height) must have existed
until the eruption of a recent, great stream of lava, as a separate islet;
a fine section, worn by the sea, shows a grand funnel-shaped mass of
basalt, surrounded by steep, sloping flanks of tuff, having in parts an
earthy, and in others a semi-resinous fracture. The tuff is traversed by
several broad, vertical dikes, with smooth and parallel sides, which I did
not doubt were formed of basalt, until I actually broke off fragments.
These dikes, however, consist of tuff like that of the surrounding strata,
but more compact, and with a smoother fracture; hence we must conclude,
that fissures were formed and filled up with the finer mud or tuff from the
crater, before its interior was occupied, as it now is, by a solidified
pool of basalt. Other fissures have been subsequently formed, parallel to
these singular dikes, and are merely filled with loose rubbish. The change
from ordinary scoriaceous particles to the substance with a semi-resinous
fracture, could be clearly followed in portions of the compact tuff of
these dikes.


At the distance of a few miles from these two craters, stands the Kicker
Rock, or islet, remarkable from its singular form. It is unstratified, and
is composed of compact tuff, in parts having the resin-like fracture. It is
probable that this amorphous mass, like that similar mass in the case first
described, once filled up the central hollow of a crater, and that its
flanks, or sloping walls, have since been worn quite away by the sea, in
which it stands exposed.


A bare, undulating tract, at the eastern end of Chatham Island, is
remarkable from the number, proximity, and form of the small basaltic
craters with which it is studded. They consist, either of a mere conical
pile, or, but less commonly, of a circle, of black and red, glossy scoriae,
partially cemented together. They vary in diameter from thirty to one
hundred and fifty yards, and rise from about fifty to one hundred feet
above the level of the surrounding plain. From one small eminence, I
counted sixty of these craters, all of which were within a third of a mile
from each other, and many were much closer. I measured the distance between
two very small craters, and found that it was only thirty yards from the
summit-rim of one to the rim of the other. Small streams of black, basaltic
lava, containing olivine and much glassy feldspar, have flowed from many,
but not from all of these craters. The surfaces of the more recent streams
were exceedingly rugged, and were crossed by great fissures; the older
streams were only a little less rugged; and they were all blended and
mingled together in complete confusion. The different growth, however, of
the trees on the streams, often plainly marked their different ages. Had it
not been for this latter character, the streams could in few cases have
been distinguished; and, consequently, this wide undulatory tract might
have (as probably many tracts have) been erroneously considered as formed
by one great deluge of lava, instead of by a multitude of small streams,
erupted from many small orifices.

In several parts of this tract, and especially at the base of the small
craters, there are circular pits, with perpendicular sides, from twenty to
forty feet deep. At the foot of one small crater, there were three of these
pits. They have probably been formed, by the falling in of the roofs of
small caverns. (M. Elie de Beaumont has described ("Mem. pour servir" etc.
tome 4 page 113) many "petits cirques d'eboulement" on Etna, of some of
which the origin is historically known.) In other parts, there are
mammiform hillocks, which resemble great bubbles of lava, with their
summits fissured by irregular cracks, which appeared, upon entering them,
to be very deep; lava has not flowed from these hillocks. There are, also,
other very regular, mammiform hillocks, composed of stratified lava, and
surmounted by circular, steep-sided hollows, which, I suppose have been
formed by a body of gas, first, arching the strata into one of the bubble-
like hillocks, and then, blowing off its summit. These several kinds of
hillocks and pits, as well as the numerous, small, scoriaceous craters, all
show that this tract has been penetrated, almost like a sieve, by the
passage of heated vapours. The more regular hillocks could only have been
heaved up, whilst the lava was in a softened state. (Sir G. Mackenzie
"Travels in Iceland" pages 389 to 392, has described a plain of lava at the
foot of Hecla, everywhere heaved up into great bubbles or blisters. Sir
George states that this cavernous lava composes the uppermost stratum; and
the same fact is affirmed by Von Buch "Descript. des Isles Canaries" page
159, with respect to the basaltic stream near Rialejo, in Teneriffe. It
appears singular that it should be the upper streams that are chiefly
cavernous, for one sees no reason why the upper and lower should not have
been equally affected at different times;--have the inferior streams flowed
beneath the pressure of the sea, and thus been flattened, after the passage
through them, of bodies of gas?)


This island consists of five, great, flat-topped craters, which, together
with the one on the adjoining island of Narborough, singularly resemble
each other, in form and height. The southern one is 4,700 feet high, two
others are 3,720 feet, a third only 50 feet higher, and the remaining ones
apparently of nearly the same height. Three of these are situated on one
line, and their craters appear elongated in nearly the same direction. The
northern crater, which is not the largest, was found by the triangulation
to measure, externally, no less than three miles and one-eighth of a mile
in diameter. Over the lips of these great, broad caldrons, and from little
orifices near their summits, deluges of black lava have flowed down their
naked sides.


Near Tagus or Banks' Cove, I examined one of these great streams of lava,
which is remarkable from the evidence of its former high degree of
fluidity, especially when its composition is considered. Near the sea-coast
this stream is several miles in width. It consists of a black, compact
base, easily fusible into a black bead, with angular and not very numerous
air-cells, and thickly studded with large, fractured crystals of glassy
albite, varying from the tenth of an inch to half an inch in diameter. (In
the Cordillera of Chile, I have seen lava very closely resembling this
variety at the Galapagos Archipelago. It contained, however, besides the
albite, well-formed crystals of augite, and the base (perhaps in
consequence of the aggregation of the augitic particles) was a shade
lighter in colour. I may here remark, that in all these cases, I call the
feldspathic crystals, "albite," from their cleavage-planes (as measured by
the reflecting goniometer) corresponding with those of that mineral. As,
however, other species of this genus have lately been discovered to cleave
in nearly the same planes with albite, this determination must be
considered as only provisional. I examined the crystals in the lavas of
many different parts of the Galapagos group, and I found that none of them,
with the exception of some crystals from one part of James Island, cleaved
in the direction of orthite or potash-feldspar.) This lava, although at
first sight appearing eminently porphyritic, cannot properly be considered
so, for the crystals have evidently been enveloped, rounded, and penetrated
by the lava, like fragments of foreign rock in a trap-dike. This was very
clear in some specimens of a similar lava, from Abingdon Island, in which
the only difference was, that the vesicles were spherical and more
numerous. The albite in these lavas is in a similar condition with the
leucite of Vesuvius, and with the olivine, described by Von Buch, as
projecting in great balls from the basalt of Lanzarote. ("Description des
Isles Canaries" page 295.) Besides the albite, this lava contains scattered
grains of a green mineral, with no distinct cleavage, and closely
resembling olivine (Humboldt mentions that he mistook a green augitic
mineral, occurring in the volcanic rocks of the Cordillera of Quito, for
olivine.); but as it fuses easily into a green glass, it belongs probably
to the augitic family: at James Island, however, a similar lava contained
true olivine. I obtained specimens from the actual surface, and from a
depth of four feet, but they differed in no respect. The high degree of
fluidity of this lava-stream was at once evident, from its smooth and
gently sloping surface, from the manner in which the main stream was
divided by small inequalities into little rills, and especially from the
manner in which its edges, far below its source, and where it must have
been in some degree cooled, thinned out to almost nothing; the actual
margin consisting of loose fragments, few of which were larger than a man's
head. The contrast between this margin, and the steep walls, above twenty
feet high, bounding many of the basaltic streams at Ascension, is very
remarkable. It has generally been supposed that lavas abounding with large
crystals, and including angular vesicles, have possessed little fluidity;
but we see that the case has been very different at Albemarle Island. (The
irregular and angular form of the vesicles is probably caused by the
unequal yielding of a mass composed, in almost equal proportion, of solid
crystals and of a viscid base. It certainly seems a general circumstance,
as might have been expected, that in lava, which has possessed a high
degree of fluidity, AS WELL AS AN EVEN-SIZED GRAIN, the vesicles are
internally smooth and spherical.) The degree of fluidity in different
lavas, does not seem to correspond with any APPARENT corresponding amount
of difference in their composition: at Chatham Island, some streams,
containing much glassy albite and some olivine, are so rugged, that they
may be compared to a sea frozen during a storm; whilst the great stream at
Albemarle Island is almost as smooth as a lake when ruffled by a breeze. At
James Island, black basaltic lava, abounding with small grains of olivine,
presents an intermediate degree of roughness; its surface being glossy, and
the detached fragments resembling, in a very singular manner, folds of
drapery, cables, and pieces of the bark of trees. (A specimen of basaltic
lava, with a few small broken crystals of albite, given me by one of the
officers, is perhaps worthy of description. It consists of cylindrical
ramifications, some of which are only the twentieth of an inch in diameter,
and are drawn out into the sharpest points. The mass has not been formed
like a stalactite, for the points terminate both upwards and downwards.
Globules, only the fortieth of an inch in diameter, have dropped from some
of the points, and adhere to the adjoining branches. The lava is vesicular,
but the vesicles never reach the surface of the branches, which are smooth
and glossy. As it is generally supposed that vesicles are always elongated
in the direction of the movement of the fluid mass, I may observe, that in
these cylindrical branches, which vary from a quarter to only the twentieth
of an inch in diameter, every air-cell is spherical.)


About a mile southward of Banks' Cove, there is a fine elliptic crater,
about five hundred feet in depth, and three-quarters of a mile in diameter.
Its bottom is occupied by a lake of brine, out of which some little
crateriform hills of tuff rise. The lower beds are formed of compact tuff,
appearing like a subaqueous deposit; whilst the upper beds, round the
entire circumference, consist of a harsh, friable tuff, of little specific
gravity, but often containing fragments of rock in layers. This upper tuff
contains numerous pisolitic balls, about the size of small bullets, which
differ from the surrounding matter, only in being slightly harder and finer
grained. The beds dip away very regularly on all sides, at angles varying,
as I found by measurement, from twenty-five to thirty degrees. The external
surface of the crater slopes at a nearly similar inclination, and is formed
by slightly convex ribs, like those on the shell of a pecten or scallop,
which become broader as they extend from the mouth of the crater to its
base. These ribs are generally from eight to twenty feet in breadth, but
sometimes they are as much as forty feet broad; and they resemble old,
plastered, much flattened vaults, with the plaster scaling off in plates:
they are separated from each other by gullies, deepened by alluvial action.
At their upper and narrow ends, near the mouth of the crater, these ribs
often consist of real hollow passages, like, but rather smaller than, those
often formed by the cooling of the crust of a lava-stream, whilst the inner
parts have flowed onward;--of which structure I saw many examples at
Chatham Island. There can be no doubt but that these hollow ribs or vaults
have been formed in a similar manner, namely, by the setting or hardening
of a superficial crust on streams of mud, which have flowed down from the
upper part of the crater. In another part of this same crater, I saw open
concave gutters between one and two feet wide, which appear to have been
formed by the hardening of the lower surface of a mud stream, instead of,
as in the former case, of the upper surface. From these facts I think it is
certain that the tuff must have flowed as mud. (This conclusion is of some
interest, because M. Dufrenoy "Mem. pour servir" tome 4 page 274, has
argued from strata of tuff, apparently of similar composition with that
here described, being inclined at angles between 18 degrees and 20 degrees,
that Monte Nuevo and some other craters of Southern Italy have been formed
by upheaval. From the facts given above, of the vaulted character of the
separate rills, and from the tuff not extending in horizontal sheets round
these crateriform hills, no one will suppose that the strata have here been
produced by elevation; and yet we see that their inclination is above 20
degrees, and often as much as 30 degrees. The consolidated strata also, of
the internal talus, as will be immediately seen, dips at an angle of above
30 degrees.) This mud may have been formed either within the crater, or
from ashes deposited on its upper parts, and afterwards washed down by
torrents of rain. The former method, in most of the cases, appears the more
probable one; at James Island, however, some beds of the friable kind of
tuff extend so continuously over an uneven surface, that probably they were
formed by the falling of showers of ashes.

Within this same crater, strata of coarse tuff, chiefly composed of
fragments of lava, abut, like a consolidated talus, against the inside
walls. They rise to a height of between one hundred and one hundred and
fifty feet above the surface of the internal brine-lake; they dip inwards,
and are inclined at an angle varying from thirty to thirty-six degrees.
They appear to have been formed beneath water, probably at a period when
the sea occupied the hollow of the crater. I was surprised to observe that
beds having this great inclination did not, as far as they could be
followed, thicken towards their lower extremities.


showing the diverging crateriform strata, and the converging stratified
talus. The highest point of these hills is 817 feet above the sea.)

This harbour occupies part of the interior of a shattered crater of tuff
larger than that last described. All the tuff is compact, and includes
numerous fragments of lava; it appears like a subaqueous deposit. The most
remarkable feature in this crater is the great development of strata
converging inwards, as in the last case, at a considerable inclination, and
often deposited in irregular curved layers. These interior converging beds,
as well as the proper, diverging crateriform strata, are represented in
Figure 13, a rude, sectional sketch of the headlands, forming this Cove.
The internal and external strata differ little in composition, and the
former have evidently resulted from the wear and tear, and redeposition of
the matter forming the external crateriform strata. From the great
development of these inner beds, a person walking round the rim of this
crater might fancy himself on a circular anticlinal ridge of stratified
sandstone and conglomerate. The sea is wearing away the inner and outer
strata, and especially the latter; so that the inwardly converging strata
will, perhaps, in some future age, be left standing alone--a case which
might at first perplex a geologist. (I believe that this case actually
occurs in the Azores, where Dr. Webster "Description" page 185, has
described a basin-formed, little island, composed of STRATA OF TUFF,
dipping inwards and bounded externally by steep sea-worn cliffs. Dr.
Daubeny supposes "Volcanoes" page 266, that this cavity must have been
formed by a circular subsidence. It appears to me far more probable, that
we here have strata which were originally deposited within the hollow of a
crater, of which the exterior walls have since been removed by the sea.)


Two craters of tuff on this island are the only remaining ones which
require any notice. One of them lies a mile and a half inland from Puerto
Grande: it is circular, about the third of a mile in diameter, and 400 feet
in depth. It differs from all the other tuff-craters which I examined, in
having the lower part of its cavity, to the height of between one hundred
and one hundred and fifty feet, formed by a precipitous wall of basalt,
giving to the crater the appearance of having burst through a solid sheet
of rock. The upper part of this crater consists of strata of the altered
tuff, with a semi-resinous fracture. Its bottom is occupied by a shallow
lake of brine, covering layers of salt, which rest on deep black mud. The
other crater lies at the distance of a few miles, and is only remarkable
from its size and perfect condition. Its summit is 1,200 feet above the
level of the sea, and the interior hollow is 600 feet deep. Its external
sloping surface presented a curious appearance from the smoothness of the
wide layers of tuff, which resembled a vast plastered floor. Brattle Island
is, I believe, the largest crater in the Archipelago composed of tuff; its
interior diameter is nearly a nautical mile. At present it is in a ruined
condition, consisting of little more than half a circle open to the south;
its great size is probably due, in part, to internal degradation, from the
action of the sea.


Fresh-water Bay.)

One side of Fresh-water Bay, in James Island, is bounded by a promontory,
which forms the last wreck of a great crater. On the beach of this
promontory, a quadrant-shaped segment of a small subordinate point of
eruption stands exposed. It consists of nine separate little streams of
lava piled upon each other; and of an irregular pinnacle, about fifteen
feet high, of reddish-brown, vesicular basalt, abounding with large
crystals of glassy albite, and with fused augite. This pinnacle, and some
adjoining paps of rock on the beach, represent the axis of the crater. The
streams of lava can be followed up a little ravine, at right angles to the
coast, for between ten and fifteen yards, where they are hidden by
detritus: along the beach they are visible for nearly eighty yards, and I
do not believe that they extend much further. The three lower streams are
united to the pinnacle; and at the point of junction (as shown in Figure
14, a rude sketch made on the spot), they are slightly arched, as if in the
act of flowing over the lip of the crater. The six upper streams no doubt
were originally united to this same column before it was worn down by the
sea. The lava of these streams is of similar composition with that of the
pinnacle, excepting that the crystals of albite appear to be more
comminuted, and the grains of fused augite are absent. Each stream is
separated from the one above it by a few inches, or at most by one or two
feet in thickness, of loose fragmentary scoriae, apparently derived from
the abrasion of the streams in passing over each other. All these streams
are very remarkable from their thinness. I carefully measured several of
them; one was eight inches thick, but was firmly coated with three inches
above, and three inches below, of red scoriaceous rock (which is the case
with all the streams), making altogether a thickness of fourteen inches:
this thickness was preserved quite uniformly along the entire length of the
section. A second stream was only eight inches thick, including both the
upper and lower scoriaceous surfaces. Until examining this section, I had
not thought it possible that lava could have flowed in such uniformly thin
sheets over a surface far from smooth. These little streams closely
resemble in composition that great deluge of lava at Albemarle Island,
which likewise must have possessed a high degree of fluidity.


In the lava and in the scoriae of this little crater, I found several
fragments, which, from their angular form, their granular structure, their
freedom from air-cells, their brittle and burnt condition, closely
resembled those fragments of primary rocks which are occasionally ejected,
as at Ascension, from volcanoes. These fragments consist of glassy albite,
much mackled, and with very imperfect cleavages, mingled with semi-rounded
grains, having tarnished, glossy surfaces, of a steel-blue mineral. The
crystals of albite are coated by a red oxide of iron, appearing like a
residual substance; and their cleavage-planes also are sometimes separated
by excessively fine layers of this oxide, giving to the crystals the
appearance of being ruled like a glass micrometer. There was no quartz. The
steel-blue mineral, which is abundant in the pinnacle, but which disappears
in the streams derived from the pinnacle, has a fused appearance, and
rarely presents even a trace of cleavage; I obtained, however, one
measurement, which proved that it was augite; and in one other fragment,
which differed from the others, in being slightly cellular, and in
gradually blending into the surrounding matrix the small grains of this
mineral were tolerably well crystallised. Although there is so wide a
difference in appearance between the lava of the little streams, and
especially of their red scoriaceous crusts, and one of these angular
ejected fragments, which at first sight might readily be mistaken for
syenite, yet I believe that the lava has originated from the melting and
movement of a mass of rock of absolutely similar composition with the
fragments. Besides the specimen above alluded to, in which we see a
fragment becoming slightly cellular, and blending into the surrounding
matrix, some of the grains of the steel-blue augite also have their
surfaces becoming very finely vesicular, and passing into the nature of the
surrounding paste; other grains are throughout, in an intermediate
condition. The paste seems to consist of the augite more perfectly fused,
or, more probably, merely disturbed in its softened state by the movement
of the mass, and mingled with the oxide of iron and with finely comminuted,
glassy albite. Hence probably it is that the fused albite, which is
abundant in the pinnacle, disappears in the streams. The albite is in
exactly the same state, with the exception of most of the crystals being
smaller in the lava and in the embedded fragments; but in the fragments
they appear to be less abundant: this, however, would naturally happen from
the intumescence of the augitic base, and its consequent apparent increase
in bulk. It is interesting thus to trace the steps by which a compact
granular rock becomes converted into a vesicular, pseudo-porphyritic lava,
and finally into red scoriae. The structure and composition of the embedded
fragments show that they are parts either of a mass of primary rock which
has undergone considerable change from volcanic action, or more probably of
the crust of a body of cooled and crystallised lava, which has afterwards
been broken up and re-liquified; the crust being less acted on by the
renewed heat and movement.


These craters, from the peculiarity of the resin-like substance which
enters largely into their composition, from their structure, their size and
number, present the most striking feature in the geology of this
Archipelago. The majority of them form either separate islets, or
promontories attached to the larger islands; and those which now stand at
some little distance from the coast are worn and breached, as if by the
action of the sea. From this general circumstance of their position, and
from the small quantity of ejected ashes in any part of the Archipelago, I
am led to conclude, that the tuff has been chiefly produced, by the
grinding together of fragments of lava within active craters, communicating
with the sea. In the origin and composition of the tuff, and in the
frequent presence of a central lake of brine and of layers of salt, these
craters resemble, though on a gigantic scale, the "salses," or hillocks of
mud, which are common in some parts of Italy and in other countries.
(D'Aubuisson "Traite de Geognosie" tome 1 page 189. I may remark, that I
saw at Terceira, in the Azores, a crater of tuff or peperino, very similar
to these of the Galapagos Archipelago. From the description given in
Freycinet "Voyage," similar ones occur at the Sandwich Islands; and

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