As to the climate of the Coal, the Ferns and the Coniferae are perhaps the two classes of plants which may be most relied upon as leading us to safe conclusions, as the genera are nearly allied to living types. All botanists admit that the abundance of ferns implies a moist atmosphere. But the coniferae, says Hooker, are of more doubtful import, as they are found in hot and dry, and in cold and dry climates; in hot and moist, and in cold and moist regions. In New Zealand the coniferae attain their maximum in numbers, constituting 1/62 part of all the flowering plants; whereas in a wide district around the Cape of Good Hope they do not form 1/1600 of the phenogamic flora. Besides the conifers, many species of ferns flourish in New Zealand, some of them arborescent, together with many lycopodiums; so that a forest in that country may make a nearer approach to the carboniferous vegetation than any other now existing on the globe.
MARINE FAUNA OF THE CARBONIFEROUS PERIOD.
It has already been stated that the Carboniferous or Mountain Limestone underlies the coal-measures in the South of England and Wales, whereas in the North, and in Scotland, marine calcareous rocks partly of the age of the Mountain Limestone alternate with shales and sandstones, containing seams of coal. In its most calcareous form the Mountain Limestone is destitute of land- plants, and is loaded with marine remains– the greater part, indeed, of the rock being made up bodily of crinoids, corals, and bryozoa with interspersed mollusca.
CORALS.
(FIGURE 474. Palaeozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA RUGOSA, Milne Edwards and Jules Haime.
a. Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); 1/2 natural size: from the Devonian of the Eifel. The lamellae are seen around the inside of the cup; the walls consist of cellular tissue; and large transverse plates, called tubulae, divide the interior into chambers. b. Arrangement of the lamellae in Polycoelia profunda, Germar, sp.; natural size: from the Magnesian Limestone, Durham. This diagram shows the quadripartite arrangement of the primary septa, characteristic of palaeozoic corals, there being four principal and eight intermediate lamellae, the whole number in this type being always a multiple of four.
c. Stauria astraeiformis, Milne Edwards. Young group, natural size. Upper Silurian, Gothland. The lamellae or septal system in each cup are divided by four prominent ridges into four groups.)
(FIGURE 475. Neozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA APOROSA, M. Edwards and J. Haime.
a. Parasmilia centralis, Mantell, sp. Vertical section; natural size. Upper Chalk, Gravesend. In this type the lamellae are massive, and extend to the axis or columella composed of loose cellular tissue, without any transverse plates like those in Figure 474, a.
b. Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault, Folkestone. In this coral the primary septa are a multiple of six. The twelve principal plates reach the columella, and between each pair there are three secondaries, in all forty-eight. The short intermediate plates which proceed from the columella are not counted. They are called pali. c. Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six primary and six secondary septa, magnified. The sextuple arrangement is always more manifest in the young than in the adult state.)
The corals deserve especial notice, as the cup-and-star corals, which have the most massive and stony skeletons, display peculiarities of structure by which they may be distinguished generally, as MM. Milne Edwards and Haime first pointed out, from all species found in strata newer than the Permian. There is, in short, an ancient or PALAEOZOIC, and a modern or NEOZOIC type, if, by the latter term, we designate (as proposed by Professor E. Forbes) all strata from the triassic to the most modern, inclusive. The accompanying diagrams (Figures 474, 475) may illustrate these types.
It will be seen that the more ancient corals have what is called a quadripartite arrangement of the chief plates or LAMELLAE– parts of the skeleton which support the organs of reproduction. The number of these lamellae in the Palaeozoic type is 4, 8, 16, etc.; while in the Neozoic type the number is 6, 12, 24, or some other multiple of six; and this holds good, whether they be simple forms, as in Figures 474, a, and 475, a, or aggregate clusters of corallites, as in 474, c. But further investigations have shown in this, as in all similar grand generalisations in natural history, that there are exceptions to the rule. Thus in the Lower Greensand Holocystis elegans (Ed. and H.) and other forms have the Palaeozoic type, and Dr. Duncan has shown to what extent the Neozoic forms penetrate downward into the Carboniferous and Devonian rocks.
(FIGURE 476. Lithostrotion basaltiforme, Phil. sp. (Lithostrotion striatum, Fleming; Astraea basaltiformis, Conyb. and Phill.). England, Ireland, Russia, Iowa, and westward of the Mississippi, United States. (D.D. Owen.)
(FIGURE 477. Lonsdaleia floriformis, Martin, sp., M. Edwards. (Lithostrotion floriforme, Fleming. Strombodes.)
a. Young specimen, with buds or corallites on the disk, illustrating calicular gemmation.
b. Part of a full-grown compound mass. Bristol, etc.; Russia.)
From a great number of lamelliferous corals met with in the Mountain Limestone, two species (Figures 476, 477) have been selected, as having a very wide range, extending from the eastern borders of Russia to the British Isles, and being found almost everywhere in each country. These fossils, together with numerous species of Zaphrentis, Amplexus, Cyathophyllum, Clisiophyllum, Syringopora, and Michelinia, form a group of rugose corals widely different from any that followed them. (For figures of these corals, see Palaeontographical Society’s Monographs 1852.)
BRYOZOA AND CRINOIDEA.
(FIGURE 478. Cyathocrinus planus, Miller. Body and arms. Mountain Limestone.)
(FIGURE 479. Cyathocrinus caryocrinoides, M’Coy. a. Surface of one of the joints of the stem. b. Pelvis or body; called also calyx or cup. c. One of the pelvic plates.)
Of the Bryozoa, the prevailing forms are Fenestella, Hemitrypa, and Polypora, and these often form considerable beds. Their net-like fronds are easily recognised. Crinoidea are also numerous in the Mountain Limestone (see Figures 478, 479), two genera, Pentremites and Codonaster, being peculiar to this formation in Europe and North America.
(FIGURE 480. Palaechinus gigas, M’Coy. Reduced one-third. Mountain Limestone. Ireland.)
In the greater part of them, the cup or pelvis, Figure 479, b, is greatly developed in size in proportion to the arms, although this is not the case in Figure 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus, and Platycrinus, are all of them characteristic of this formation. Other Echinoderms are rare, a few Sea-Urchins only being known: these have a complex structure, with many more plates on their surface than are seen in the modern genera of the same group. One genus, the Palaechinus (Figure 480), is the analogue of the modern Echinus, but has four, five, or six rows of plates in the interambulacral region or area, whereas the modern genera have only two. The other, Archaeocidaris, represents, in like manner, the Cidaris of the present seas.
MOLLUSCA.
(FIGURE 481. Productus semireticulatus, Martin, sp. (P. antiquatus, Sowerby.) Mountain Limestone. England, Russia, the Andes, etc.)
(FIGURE 482. Spirifera trigonalis, Martin, sp. Mountain Limestone. Derbyshire, etc.)
(FIGURE 483. Spirifera glabra, Martin, sp. Mountain Limestone.)
The British Carboniferous mollusca enumerated by Mr. Etheridge comprise 653 species referable to 86 genera, occurring chiefly in the Mountain Limestone. (Quarterly Geological Journal volume 23 page 674 1867.) Of this large number only 40 species are common to the underlying Devonian rocks, 9 of them being Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or Palliobranchiates). This latter group constitutes the larger part of the Carboniferous Mollusca, 157 species being known in Great Britain alone, and it will be found to increase in importance in the fauna of the primary rocks the lower we descend in the series. Perhaps the most characteristic shells of the formation are large species of Productus, such as P. giganteus, p. hemisphericus, P. semireticulatus (Figure 481), and P. scabriculus. Large plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Figure 482), also abound; and smooth species, such as Spirifera glabra (Figure 483), with its numerous varieties.
(FIGURE 484. Terebratula hastata, Sowerby, with radiating bands of colour. Mountain Limestone. Derbyshire, Ireland, Russia, etc.)
(FIGURE 485. Aviculopecten sublobatus, Phill. Mountain Limestone. Derbyshire, Yorkshire.)
(FIGURE 486. Pleurotomaria carinata, Sowerby. (P. flammigera, Phillips). Mountain Limestone. Derbyshire, etc.)
Among the brachiopoda, Terebratula hastata (Figure 484) deserves mention, not only for its wide range, but because it often retains the pattern of the original coloured stripes which ornamented the living shell. These coloured bands are also preserved in several lamellibranchiate bivalves, as in Aviculopecten (Figure 485), in which dark stripes alternate with a light ground. In some also of the spiral univalves the pattern of the original painting is distinctly retained, as in Pleurotomaria (Figure 486), which displays wavy blotches, resembling the colouring in many recent trochidae.
(FIGURE 487. Euomphalus pentangulatus, Sowerby. Mountain Limestone. a. Upper side.
b. Lower or umbilical side.
c. View showing mouth, which is less pentagonal in older individuals. d. View of polished section, showing internal chambers.)
Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but the majority, though often referred to as living types, such as Isocardia, Turritella, and Buccinum, belong really to forms which appear to have become extinct at the close of the Palaeozoic epoch. Euomphalus is a characteristic univalve shell of this period. In the interior it is divided into chambers (Figure 487, d), the septa or partitions not being perforated as in foraminiferous shells, or in those having siphuncles, like the Nautilus. The animal appears to have retreated at different periods of its growth from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl. The animal of the recent Turritella communis partitions off in like manner as it advances in age a part of its spire, forming a shelly septum.
(FIGURE 488. Bellerophon costatus, Sowerby. Mountain Limestone.)
More than twenty species of the genus Bellerophon (see Figure 488), a shell like the living Argonaut without chambers, occur in the Mountain Limestone. The genus is not met with in strata of later date. It is most generally regarded as belonging to the pelagic Nucleobranchiata and the family Atlantidae, partly allied to the Glass-Shell, Carinaria; but by some few it is thought to be a simple form of Cephalopod.
(FIGURE 489. Portion of Orthoceras laterale. Phill. Mountain Limestone.)
(FIGURE 490. Goniatites crenistra, Phillips. Mountain Limestone. North America, Britain, Germany, etc.
a. Lateral view.
b. Front view, showing the mouth.)
The carboniferous Cephalopoda do not depart so widely from the living type (the Nautilus) as do the more ancient Silurian representatives of the same order; yet they offer some remarkable forms. Among these is Orthoceras, a siphuncled and chambered shell, like a Nautilus uncoiled and straightened (Figure 489). Some species of this genus are several feet long. The Goniatite is another genus, nearly allied to the Ammonite, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is angular, continuous, and uninterrupted. The species represented in Figure 490 is found in most localities, and presents the zigzag character of the septal lobes in perfection. The dorsal position of the siphuncle, however, clearly distinguishes the Goniatite from the Nautilus, and proves it to have belonged to the family of the Ammonites, from which, indeed, some authors do not believe it to be generically distinct.
FOSSIL FISH.
(FIGURE 491. Psammodus porosus, Agassiz. Bone-bed, Mountain Limestone. Bristol, Armagh.)
(FIGURE 492. Cochliodus contortus, Agassiz. Bone-bed, Mountain Limestone. Bristol, Armagh.)
The distribution of these is singularly partial; so much so, that M. De Koninck of Liege, the eminent palaeontologist, once stated to me that, in making his extensive collection of the fossils of the Mountain Limestone of Belgium, he had found no more than four or five examples of the bones or teeth of fishes. Judging from Belgian data, he might have concluded that this class of vertebrata was of extreme rarity in the Carboniferous seas; whereas the investigation of other countries has led to quite a different result. Thus, near Clifton, on the Avon, as well as at numerous places around the Bristol basin from the Mendip Hills to Tortworth, there is a celebrated “bone-bed,” almost entirely made up of ichthyolites. It occurs at the base of the Lower Limestone shales immediately resting upon the passage beds of the Old Red Sandstone. Similar bone-beds occur in the Carboniferous Limestone of Armagh, in Ireland, where they are made up chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled as if drifted from a distance. Some teeth are sharp and pointed, as in ordinary sharks, of which the genus Cladodus afford an illustration; but the majority, as in Psammodus and Cochliodus, are, like the teeth of the Cestracion of Port Jackson (see Figure 261), massive palatal teeth fitted for grinding. (See Figures 491, 492.)
There are upward of seventy other species of fossil fish known in the Mountain Limestone of the British Islands. The defensive fin-bones of these creatures are not infrequent at Armagh and Bristol; those known as Oracanthus, Ctenocanthus, and Onchus are often of a very large size. Ganoid fish, such as Holoptychius, also occur; but these are far less numerous. The great Megalichthys Hibberti appears to range from the Upper Coal-measures to the lowest Carboniferous strata.
FORAMINIFERA.
(FIGURE 493. Fusulina cylindrica, d’Orbigny. Magnified 3 diameters. Mountain Limestone.)
In the upper part of the Mountain Limestone group in the south-west of England, near Bristol, limestones having a distinct oolitic structure alternate with shales. In these rocks the nucleus of every minute spherule is seen, under the microscope, to consist of a small rhizopod or foraminifer. This division of the lower animals, which is represented so fully at later epochs by the Nummulites and their numerous minute allies, appears in the Mountain Limestone to be restricted to a very few species, among which Textularia, Nodosaria, Endothyra, and Fusulina (Figure 493), have been recognised. The first two genera are common to this and all the after periods; the third has been found in the Upper Silurian, but is not known above the Carboniferous strata; the fourth (Figure 493) is characteristic of the Mountain Limestone in the United States, Arctic America, Russia, and Asia Minor, but is also known in the Permian.
CHAPTER XXV.
DEVONIAN OR OLD RED SANDSTONE GROUP.
Classification of the Old Red Sandstone in Scotland and in Devonshire. Upper Old Red Sandstone in Scotland, with Fish and Plants. Middle Old Red Sandstone.
Classification of the Ichthyolites of the Old Red, and their Relation to Living Types.
Lower Old Red Sandstone, with Cephalaspis and Pterygotus. Marine or Devonian Type of Old Red Sandstone. Table of Devonian Series.
Upper Devonian Rocks and Fossils.
Middle.
Lower.
Eifel Limestone of Germany.
Devonian of Russia.
Devonian Strata of the United States and Canada. Devonian Plants and Insects of Canada.
CLASSIFICATION OF THE TWO TYPES OF OLD RED SANDSTONE.
We have seen that the Carboniferous strata are surmounted by the Permian and Trias, both originally included in England under the name “New Red Sandstone,” from the prevailing red colour of the strata. Under the coal came other red sandstones and shales which were distinguished by the title of “Old Red Sandstone.” Afterwards the name of “Devonian” was given by Sir R. Murchison and Professor Sedgwick to marine fossiliferous strata which, in the south of England, occupy a similar position between the overlying coal and the underlying Silurian formations.
It may be truly said that in the British Isles the rocks of this age present themselves in their mineral aspect, and even to some extent in their fossil contents, under two very different forms; the one as distinct from the other as are often lacustrine or fluviatile from marine strata. It has indeed been suggested that by far the greater part of the deposits belonging to what may be termed the Old Red Sandstone type are of fresh-water origin. The number of land- plants, the character of the fishes, and the fact that the only shell yet discovered belongs to the genus Anodonta, must be allowed to lend no small countenance to this opinion. In this case the difficulty of classification when the strata of this type are compared in different regions, even where they are contiguous, may arise partly from their having been formed in distinct hydrographical basins, or in the neighbourhood of the land in shallow parts of the sea into which large bodies of fresh-water entered, and where no marine mollusca or corals could flourish. Under such geographical conditions the limited extent of some kinds of sediment, as well as the absence of those marine forms by which we are able to identify or contrast marine formations, may be explained, while the great thickness of the rocks, which might seem at first sight to require a corresponding depth of water, can often be shown to have been due to the gradual sinking down of the bottom of the estuary or sea where the sediment was accumulated.
Another active cause of local variation in Scotland was the frequency of contemporaneous volcanic eruptions; some of the rocks derived from this source, as between the Grampians and the Tay, having formed islands in the sea, and having been converted into shingle and conglomerate, before the upper portions of the red shales and sandstones were superimposed.
The dearth of calcareous matter over wide areas is characteristic of the Old Red Sandstone. This is, no doubt, in great part due to the absence of shells and corals; but why should these be so generally wanting in all sedimentary rocks the colour of which is determined by the red oxide of iron? Some geologists are of opinion that the waters impregnated with this oxide were prejudicial to living beings, others that strata permeated with this oxide would not preserve such fossil remains.
In regard to the two types, the Old Red Sandstone and the Devonian, I shall first treat of them separately, and then allude to the proofs of their having been to a great extent contemporaneous. That they constitute a series of rocks intermediate in date between the lowest Carboniferous and the uppermost Silurian is not disputed by the ablest geologists; and it can no longer be contended that the Upper, Middle, and Lower Old Red Sandstone preceded in date the three divisions to which, by aid of the marine shells, the Devonian rocks have been referred, while, on the other hand, we have not yet data for enabling us to affirm to what extent the subdivisions of the one series may be the equivalents in time of those of the other.
UPPER OLD RED SANDSTONE.
(FIGURE 494. Anodonta Jukesii, Forbes. Upper Devonian, Kiltorkan, Ireland.)
(FIGURE 495. Bifurcating branch of Lepidodendron Griffithsii, Brongn. Upper Devonian, Kilkenny.)
(FIGURE 496. Palaeopteris Hibernica, Schimp. (Cyclopteris Hibernica), Edward Forbes (Adiantites, Gop.). Upper Devonian, Kilkenny.)
The highest beds of the series in Scotland, lying immediately below the coal in Fife, are composed of yellow sandstone well seen at Dura Den, near Coupar, in Fife, where, although the strata contain no mollusca, fish have been found abundantly, and have been referred to the genera Holoptychius, Pamphractus, Glyptopomus, and many others. In the county of Cork, in Ireland, a similar yellow sandstone occurs containing fish of genera characteristic of the Scotch Old Red Sandstone, as for example Coccosteus (a form represented by many species in the Old Red Sandstone and by one only in the Carboniferous group), and Glytolepis and Asterolepis, both exclusively confined to the “Old Red.” In the same Irish sandstone at Kiltorkan has been found an Anodonta or fresh-water mussel, the only shell hitherto discovered in the Old Red Sandstone of the British Isles (see Figure 494). In the same formation are found the fern (Figure 496) and the Lepidodendron (Figure 495), and other species of plants, some of which, Professor Heer remarks, agree specifically with species from the lower carboniferous beds. This induces him to lean to the opinion long ago advocated by Sir Richard Griffiths, that the yellow sandstone, in spite of its fish remains, should be classed as Lower Carboniferous, an opinion which I am not yet prepared to adopt. Between the Mountain Limestone and the yellow sandstone in the south-west of Ireland there intervenes a formation no less than 5000 feet thick, called the “Carboniferous slate,” and at the base of this, in some places, are local deposits, such as the Glengariff Grits, which appear to be beds of passage between the Carboniferous and Old Red Sandstone groups.
It is a remarkable result of the recent examination of the fossil flora of Bear Island, latitude 74 degrees 30′ N., that Professor Heer has described as occurring in that part of the Arctic region (nearly twenty-six degrees to the north of the Irish locality) a flora agreeing in several of its species with that of the yellow sandstones of Ireland. This Bear Island flora is believed by Professor Heer to comprise species of plants some of which ascend even to the higher stages of the European Carboniferous formation, or as high as the Mountain Limestone and Millstone Grit. Palaeontologists have long maintained that the same species which have a wide range in space are also the most persistent in time, which may prepare us to find that some plants having a vast geographical range may also have endured from the period of the Upper Devonian to that of the Millstone Grit.
(FIGURE 497. Scale of Holoptychius nobilissimus, Agassiz. Clashbinnie. 1/2 natural size.)
(FIGURE 498. Holoptychius, as restored by Professor Huxley. a. The fringed pectoral fins.
b. The fringed ventral fins.
c. Anal fin.
d, e. Dorsal fins.)
Outliers of the Upper “Old Red” occur unconformably on older members of the group, and the formation represented at Whiteness, near Arbroath, a, Figure 55, may probably be one of these outliers, though the want of organic remains renders this uncertain. It is not improbable that the beds given in this section as Nos. 1, 2, and 3, may all belong to the early part of the period of the Upper Old Red, as some scales of Holoptychius nobilissimus have been found scattered through these beds, No. 2, in Strathmore. Another nearly allied Holoptychius occurs in Dura Den, see Figure 498 of this fish and also Figure 497 of one of its scales, as these last are often the only parts met with; being scattered in Forfarshire through red-coloured shales and sandstones, as are scales of a large species of the same genus in a corresponding matrix in Herefordshire. (Siluria 4th edition page 265.) The number of fish obtained from the British Upper Old Red Sandstone amounts to fifteen species referred to eleven genera.
Sir R. Murchison groups with this upper division of the Old Red of Scotland certain light-red and yellow sandstones and grits which occur in the northernmost part of the mainland, and extend also into the Orkney and Shetland Islands. They contain Calamites and other plants which agree generically with Carboniferous forms.
MIDDLE OLD RED SANDSTONE.
In the northern part of Scotland there occur a great series of bituminous schists and flagstones, to the fossil fish of which attention was first called by the late Hugh Miller. They were afterwards described by Agassiz, and the rocks containing them were examined by Sir R. Murchison and Professor Sedgwick, in Caithness, Cromarty, Moray, Nairn, Gamrie in Banff, and the Orkneys and Shetlands, in which great numbers of fossil fish have been found. These were at first supposed to be the oldest known vertebrate animals, as in Cromarty the beds in which they occur seem to form the base of the Old Red system resting almost immediately on the crystalline or metamorphic rocks. But in fact these fish-bearing beds, when they are traced from north to south, or to the central parts of Scotland, thin out, so that their relative age to the Lower Old Red Sandstone, presently to be mentioned, was not at first detected, the two formations not appearing in superposition in the same district. In Caithness, however, many hundred feet below the fish-zone of the middle division, remains of Pteraspis were found by Mr. Peach in 1861. This genus has never yet been found in either of the two higher divisions of the Old Red Sandstone, and confirms Sir R. Murchison’s previous suspicion that the rocks in which it occurs belong to the Lower “Old Red,” or agree in age with the Arbroath paving-stone. (Siluria 4th edition page 258.)
FOSSIL FISH OF THE MIDDLE OLD RED SANDSTONE.
The Devonian fish were referred by Agassiz to two of his great orders, namely, the Placoids and Ganoids. Of the first of these, which in the Recent period comprise the shark, the dog-fish, and the ray, no entire skeletons are preserved, but fin-spines, called ichthyodorulites, and teeth occur. On such remains the genera Onchus, Odontacanthus, and Ctenodus, a supposed cestraciont, and some others, have been established.
(FIGURE 499. Polypterus. See Agassiz, “Recherces sur les Poissons Fossiles.” Living in the Nile and other African rivers. a. One of the fringed pectoral fins.
b. One of the ventral fins.
c. Anal fin.
d. Dorsal fin, or row of finlets.)
(FIGURE 500. Restoration of Osteolepis. Pander. Old Red Sandstone, or Devonian. a. One of the fringed pectoral fins.
b. One of the ventral fins.
c. Anal fin.
d, e. Dorsal fins.)
By far the greater number of the Old Red Sandstone fishes belong to a sub-order of Ganoids instituted by Huxley in 1861, and for which he has proposed the name of Crossopterygidae (Abridged from crossotos, a fringe, and pteryx, a fin.), or the fringe-finned, in consideration of the peculiar manner in which the fin-rays of the paired fins are arranged so as to form a fringe round a central lobe, as in the Polypterus (see a, Figure 499), a genus of which there are several species now inhabiting the Nile and other African rivers. The reader will at once recognise in Osteolepis (Figure 500), one of the common fishes of the Old Red Sandstone, many points of analogy with Polypterus. They not only agree in the structure of the fin, at first pointed out by Huxley, but also in the position of the pectoral, ventral, and anal fins, and in having an elongated body and rhomboidal scales. On the other hand, the tail is more symmetrical in the recent fish, which has also an apparatus of dorsal finlets of a very abnormal character, both as to number and structure. As to the dorsals of Osteolepis, they are regular in structure and position, having nothing remarkable about them, except that there are two of them, which is comparatively unusual in living fish.
Among the “fringe-finned” Ganoids we find some with rhomboidal scales, such as Osteolepis, Figure 500; others with cycloidal scales, as Holoptychius, before mentioned (see Figure 498). In the genera Dipterus and Diplopterus, as Hugh Miller pointed out, and in several other of the fringe-finned genera, as in Gyroptychius and Glyptolepis, the two dorsals are placed far backward, or directly over the ventral and anal fins. The Asterolepis was a ganoid fish of gigantic dimensions. A. Asmusii, Eichwald, a species characteristic of the Old Red Sandstone of Russia, as well as that of Scotland, attained the length of between twenty and thirty feet. It was clothed with strong bony armour, embossed with star-like tubercles, but it had only a cartilaginous skeleton. The mouth was furnished with two rows of teeth, the outer ones small and fish-like, the inner larger and with a reptilian character. The Asterolepis occurs also in the Devonian rocks of North America.
If we except the Placoids already alluded to, and a few other families of doubtful affinities, all the Old Red Sandstone fishes are Ganoids, an order so named by Agassiz from the shining outer surface of their scales; but Professor Huxley has also called our attention to the fact that, while a few of the primary and the great majority of the secondary Ganoids resemble the living bony pike, Lepidosteus, or the Amia, genera now found in North American rivers, and one of them, Lepidosteus, extending as far south as Guatemala, the Crossopterygii, or fringe-finned Ichthyolites, of the Old Red are closely related to the African Polypterus, which is represented by five or six species now inhabiting the Nile and the rivers of Senegal. These North American and African Ganoids are quite exceptional in the living creation; they are entirely confined to the northern hemisphere, unless some species of Polypterus range to the south of the line in Africa; and, out of about 9000 living species of fish known to M. Gunther, and of which more than 6000 are now preserved in the British Museum, they probably constitute no more than nine.
If many circumstances favour the theory of the fresh-water origin of the Old Red Sandstone, this view of its nature is not a little confirmed by our finding that it is in Llake Superior and the other inland Canadian seas of fresh water, and in the Mississippi and African rivers, that we at present find those fish which have the nearest affinity to the fossil forms of this ancient formation.
(FIGURE 501. Pterichthys, Agassiz; Upper side, showing mouth; as restored by H. Miller.)
Among the anomalous forms of Old Red fishes not referable to Huxley’s Crossopterygii is the Pterichthys, of which five species have been found in the middle division of the Old Red of Scotland. Some writers have compared their shelly covering to that of Crustaceans, with which, however, they have no real affinity. The wing-like appendages, whence the genus is named, were first supposed by Hugh Miller to be paddles, like those of the turtle; and there can now be no doubt that they do really correspond with the pectoral fins.
The number of species of fish already obtained from the middle division of the Old Red Sandstone in Great Britain is about 70, and the principal genera, besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis, Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides.
LOWER OLD RED SANDSTONE.
(FIGURE 502. Cephalaspis Lyellii, Agassiz. Length 6 3/4 inches. From a specimen in my collection found at Glammiss, in Forfarshire. (See other figures, Agassiz, volume 2 table 1 a and 1 b.
a. One of the peculiar scales with which the head is covered when perfect. These scales are generally removed, as in the specimen above figured. b, c. Scales from different parts of the body and tail.)
The third or lowest division south of the Grampians consists of grey paving- stone and roofing-slate, with associated red and grey shales; these strata underlie a dense mass of conglomerate. In these grey beds several remarkable fish have been found of the genus named by Agassiz Cephalaspis, or “buckler- headed,” from the extraordinary shield which covers the head (see Figure 502), and which has often been mistaken for that of a trilobite, such as Asaphus. A species of Pteraspis, of the same family, has also been found by the Reverend Hugh Mitchell in beds of corresponding age in Perthshire; and Mr. Powrie enumerates no less than five genera of the family Acanthodidae, the spines, scales, and other remains of which have been detected in the grey flaggy sandstones. (Powrie Geological Quarterly Journal volume 20 page 417.)
(FIGURE 503. Pterygotus anglicus, Agassiz. Middle portion of the back of the head called the seraphim.)
(FIGURE 504. Pterygotus anglicus, Agassiz. Forfarshire. Ventral aspect. Restored by H. Wodward, F.G.S.
a. Carapace, showing the large sessile eyes at the anterior angles. b. The metastoma or post-oral plate (serving the office of a lower lip). c, c. Chelate appendages (antennules).
d. First pair of simple palpi (antennae). e. Second pair of simple palpi (mandibles). f. Third pair of simple palpi (first maxillae). g. Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillae.
h. Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line.
1-6. Thoracic segments.
7-12. Abdominal segments.
13. Telson, or tail-plate.)
In the same formation at Carmylie, in Forfarshire, commonly known as the Arbroath paving-stone, fragments of a huge crustacean have been met with from time to time. They are called by the Scotch quarrymen the “Seraphim,” from the wing-like form and feather-like ornament of the thoracic appendage, the part most usually met with. Agassiz, having previously referred some of these fragments to the class of fishes, was the first to recognise their crustacean character, and, although at the time unable correctly to determine the true relation of the several parts, he figured the portions on which he founded his opinion, in the first plate of his “Poissons Fossiles du Vieux Gres Rouge.”
A restoration in correct proportion to the size of the fragments of P. anglicus (Figure 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire, would give us a creature measuring from five to six feet in length, and more than one foot across.
The largest crustaceans living at the present day are the Inachus Kaempferi, of De Haan, from Japan (a brachyurous or short-tailed crab), chiefly remarkable for the extraordinary length of its limbs; the fore-arm measuring four feet in length, and the others in proportion, so that it covers about 25 square feet of ground; and the Limulus Moluccanus, the great King Crab of China and the Eastern seas, which, when adult, measures 1 1/2 foot across its carapace, and is three feet in length.
(FIGURE 505. Parka decipiens, Fleming. In sandstone of lower beds of Old Red, Ley’s Mill, Forfarshire.)
(FIGURE 506. Parka decipiens, Fleming. In shale of Lower Old Red, Park Hill, Fife.)
(FIGURE 507. Shale of Old Red Sandstone. Forfarshire. With impression of plants and eggs of Crustaceans.
a. Two pair of ova? resembling those of large Salamanders or Tritons– on the same leaf.
b, b. Detached ova.)
Besides some species of Pterygotus, several of the allied genus Eurypterus occur in the Lower Old Red Sandstone, and with them the remains of grass-like plants so abundant in Forfarshire and Kincardineshire as to be useful to the geologist by enabling him to identify the inferior strata at distant points. Some botanists have suggested that these plants may be of the family Fluviales, and of fresh-water genera. They are accompanied by fossils, called “berries” by the quarrymen, which they compared to a compressed blackberry (see Figures 505, 506), and which were called “Parka” by Dr. Fleming. They are now considered by Mr. Powrie to be the eggs of crustaceans, which is highly probable, for they have not only been found with Pterygotus anglicus in Forfarshire and Perthshire, but also in the Upper Silurian strata of England, in which species of the same genus, Pterygotus, occur.
The grandest exhibitions, says Sir R. Murchison, of the Old Red Sandstone in England and Wales appear in the escarpments of the Black Mountains and in the Fans of Brecon and Carmarthen, the one 2862, and the other 2590 feet above the sea. The mass of red and brown sandstone in these mountains is estimated at not less than 10,000 feet, clearly intercalated between the Carboniferous and Silurian strata. No shells or corals have ever been found in the whole series, not even where the beds are calcareous, forming irregular courses of concretionary lumps called “corn-stones,” which may be described as mottled red and green earthy limestones. The fishes of this lowest English Old Red are Cephalaspis and Pteraspis, specifically different from species of the same genera which occur in the uppermost Ludlow or Silurian tilestones. Crustaceans also of the genus Eurypterus are met with.
MARINE OR DEVONIAN TYPE.
We may now speak of the marine type of the British strata intermediate between the Carboniferous and Silurian, in treating of which we shall find it much more easy to identify the Upper, Middle, and Lower divisions with strata of the same age in other countries. It was not until the year 1836 that Sir R. Murchison and Professor Sedgwick discovered that the culmiferous or anthracitic shales and sandstones of North Devon, several thousand feet thick, belonged to the coal, and that the beds below them, which are of still greater thickness, and which, like the carboniferous strata, had been confounded under the general name “graywacke,” occupied a geological position corresponding to that of the Old Red Sandstone already described. In this reform they were aided by a suggestion of Mr. Lonsdale, who, after studying the Devonshire fossils, perceived that they belonged to a peculiar palaeontological type of intermediate character between the Carboniferous and Silurian.
It is in the north of Devon that these formations may best be studied, where they have been divided into an Upper, Middle, and Lower Group, and where, although much contorted and folded, they have for the most part escaped being altered by intrusive trap-rocks and by granite, which in Dartmoor and the more southern parts of the same county have often reduced them to a crystalline or metamorphic state.
TABLE 25.1 DEVONIAN SERIES IN NORTH DEVON.
UPPER DEVONIAN OR PILTON GROUP.
a. Sandy slates and schists with fossils, 36 species out of 110 common to the Carboniferous group (Pilton, Barnstaple, etc.), resting on soft schists in which fossils are very abundant (Croyde, etc.), and which pass down into
b. Yellow, brown, and red sandstone, with land plants (Cyclopteris, etc.) and marine shells. One zone, characterised by the abundance of cucullaea (Baggy Point, Marwood, Sloly, etc.) resting on hard grey and reddish sandstone and micaceous flags, no fossils yet found (Dulverton, Pickwell, Down, etc.)
MIDDLE DEVONIAN OR ILFRACOMBE GROUP.
a. Green glossy slates of considerable thickness, no fossils yet recorded from these beds (Mortenoe, Lee Bay, etc.).
b. Slates and schists, with several irregular courses of limestone containing shells and corals like those of the Plymouth Limestone (Combe Martin, Ilfracombe, etc.).
LOWER DEVONIAN OR LYNTON GROUP.
a. Hard, greenish, red, and purple sandstone– no fossils yet found (Hangman Hill, etc.).
b. Soft slates with subordinate sandstones– fossils numerous at various horizons– Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.).
Table 25.1 exhibits the sequence of the strata or subdivisions as seen both on the sea-coast of the British Channel and in the interior of Devon. It will be seen that in all main points it agrees with the table drawn up in 1864 for the sixth edition of my “Elements.” Mr. Etheridge has since published an excellent account of the different subdivisions of the rocks and their fossils, and has also pointed out their relation to the corresponding marine strata of the Continent. (Quarterly Geological Journal volume 23 1867.) The slight modifications introduced in my table since 1864 are the result of a tour made in 1870 in company with Mr. T. Mck. Hughes, when we had the advantage of Mr. Etheridge’s memoir as our guide.
The place of the sandstones of the Foreland is not yet clearly made out, as they are cut off by a great fault and disturbance.
UPPER DEVONIAN ROCKS.
(FIGURE 508. Spirifera disjuncta, Sowerby. Syn. Sp. Verneuilii, Murch. Upper Devonian, Boulogne.)
(FIGURE 509. Phacops latifrons, Bronn. Characteristic of the Devonian in Europe, Asia, and N. and S. America.)
(FIGURE 510. Clymenia linearis, Munster. Petherwyn, Cornwall; Elbersreuth, Bavaria.)
(FIGURE 511. Cypridina serrato-striata, Sandberger, Weilburg, etc.; Cornwall, Nassau, Saxony, Belgium.)
The slates and sandstones of Barnstaple (a and b of the preceding section) contain the shell Spirifera disjuncta, Sowerby (S. Verneuilii, Murch.), (see Figure 508), which has a very wide range in Europe, Asia Minor, and even China; also Strophalosia caperata, together with the large trilobite Phacops latifrons, Bronn. (See Figure 509), which is all but world-wide in its distribution. The fossils are numerous, and comprise about 150 species of mollusca, a fifth of which pass up into the overlying Carboniferous rocks. To this Upper Devonian belong a series of limestones and slates well developed at Petherwyn, in Cornwall, where they have yielded 75 species of fossils. The genus of Cephalopoda called Clymenia (Figure 510) is represented by no less than eleven species, and strata occupying the same position in Germany are called Clymenien- Kalk, or sometimes Cypridinen-Schiefer, on account of the number of minute bivalve shells of the crustacean called Cypridina serrato-striata (Figure 511), which is found in these beds, in the Rhenish provinces, the Harz, Saxony, and Silesia, as well as in Cornwall and Belgium.
MIDDLE DEVONIAN ROCKS.
(FIGURE 512. Heliolites porosa, Goldf. sp. (Porites pyriformis, Lonsd.) a. Portion of the same magnified. Middle Devonian, Torquay, Plymouth; Eifel.)
(FIGURE 513. Favosites cervicornis, Blainv. S. Devon, from a polished specimen. a. Portion of the same magnified, to show the pores.)
(FIGURE 514. Cyathophyllum caespitosum, Goldf.; Plymouth and Ilfracombe. b. A terminal star.
c. Vertical section, exhibiting transverse plates, and part of another branch.)
We come next to the most typical portion of the Devonian system, including the great limestones of Plymouth and Torbay, replete with shells, trilobites, and corals. Of the corals 51 species are enumerated by Mr. Etheridge, none of which pass into the Carboniferous formation. Among the genera we find Favosites, Heliolites, and Cyathophyllum. The two former genera are very frequent in Silurian rocks: some few even of the species are said to be common to the Devonian and Silurian groups, as, for example, Favosites cervicornis (Figure 513), one of the commonest of all the Devonshire fossils. The Cyathophyllum caespitosum (Figure 514) and Heliolites pyriformis (Figure 512) are species peculiar to this formation.
(FIGURE 515. Stringocephalus Burtini, Def. a. Valves united.
b. Interior of ventral or large valve, showing thick partition and portion of a large process which projects from the dorsal valve across the shell.)
(FIGURE 516. Uncites Gryphus, Def. Middle Devonian. S. Devon and the Continent.)
With the above are found no less than eleven genera of stone-lilies or crinoids, some of them, such as Cupressocrinites, distinct from any Carboniferous forms. The mollusks, also, are no less characteristic; of 68 species of Brachiopoda, ten only are common to the Carboniferous Limestone. The Stringocephalus Burtini (Figure 515) and Uncites Gryphus (Figure 516) may be mentioned as exclusively Middle Devonian genera, and extremely characteristic of the same division in Belgium. The Stringocephalus is also so abundant in the Middle Devonian of the banks of the Rhine as to have suggested the name of Stringocephalus Limestone. The only two species of Brachiopoda common to the Silurian and Devonian formations are Atrypa reticularis (Figure 532), which seems to have been a cosmopolite species, and Strophomena rhomboidalis.
(FIGURE 517. Megalodon cucullatus, Sowerby. Eifel; also Bradley, S. Devon. a. The valves united.
b. Interior of valve, showing the large cardinal tooth.)
(FIGURE 518. Conularia ornata, D’Arch. and De Vern. (Geological Transactions Sec. Ser. volume 6. Plate 29.) Refrath, near Cologne.)
(FIGURE 519. Bronteus flabellifer, Goldf. Mid. Devon; S. Devon; and the Eifel.)
Among the peculiar lamellibranchiate bivalves common to the Plymouth limestone of Devonshire and the Continent, we find the Megalodon (Figure 517). There are also twelve genera of Gasteropods which have yielded 36 species, four of which pass to the Carboniferous group, namely Macrocheilus, Acroculia, Euomphalus, and Murchisonia. Pteropods occur, such as Conularia (Figure 518), and Cephalopods, such as Cyrtoceras, Gyroceras, Orthoceras, and others, nearly all of genera distinct from those prevailing in the Upper Devonian Limestone, or Clymenien- kalk of the Germans already mentioned. Although but few species of Trilobites occur, the characteristic Bronteus flabellifer (Figure 519) is far from rare, and all collectors are familiar with its fan-like tail. In this same group, called, as before stated, the Stringocephalus, or Eifel Limestone, in Germany, several fish remains have been detected, and among others the remarkable genus Coccosteus, covered with its tuberculated bony armour; and these ichthyolites serve, as Sir R. Murchison observes (Siluria page 362), to identify this middle marine Devonian with the Old Red Sandstone of Britain and Russia.
(FIGURE 520. Calceola sandalina, Lam. Eifel; also South Devon. a. Ventral valve.
b. Inner side of dorsal valve.)
Beneath the Eifel Limestone (the great central and typical member of “the Devonian” on the Continent) lie certain schists called by German writers “Calceola-schiefer,” because they contain in abundance a fossil body of very curious structure, Calceola sandalina (Figure 520), which has been usually considered a brachiopod, but which some naturalists have lately referred to a Goniophyllum, supposing it to be an abnormal form of the order Zoantharia rugosa (see Figure 474), differing from all other corals in being furnished with a strong operculum. This is by no means a rare fossil in the slaty limestone of South Devon, and, like the Eifel form, is confined to the middle group of this country.
LOWER DEVONIAN ROCKS.
(FIGURE 521. Spirifera mucronata, Hall. Devonian of Pennsylvania.)
A great series of sandstones and glossy slates, with Crinoids, Brachiopods, and some corals, occurring on the coast at Lynmouth and the neighbourhood, and called the Lynton Group (see Table 25.1), form the lowest member of the Devonian in North Devon. Among the 18 species of all classes enumerated by Mr. Etheridge, two-thirds are common to the Middle Devonian, but only one, the ubiquitous Atrypa reticularis, can with certainty be identified with Silurian species. Among the characteristic forms are Alveolites suborbicularis, also common to this formation in the Rhine, and Orthis arcuata, very widely spread in the North Devon localities. But we may expect a large addition to the number of fossils whenever these strata shall have been carefully searched. The Spirifer Sandstone of Sandberger, as exhibited in the rocks bordering the Rhine between Coblentz and Caub, belong to this Lower division, and the same broad-winged Spirifers distinguish the Devonian strata of North America.
(FIGURE 522. Homalonotus armatus, Burmeister. Lower Devonian; Daun, in the Eifel; and S. Devon.
Obs. The two rows of spines down the body give an appearance of more distinct trilobation than really occurs in this or most other species of the genus.)
Among the Trilobites of this era several large species of Homalonotus (Figure 522) are conspicuous. The genus is still better known as a Silurian form, but the spinose species appear to belong exclusively to the “Lower Devonian,” and are found in Britain, Europe, and the Cape of Good Hope.
DEVONIAN OF RUSSIA.
The Devonian strata of Russia extend, according to Sir R. Murchison, over a region more spacious than the British Isles; and it is remarkable that, where they consist of sandstone like the “Old Red” of Scotland and Central England, they are tenanted by fossil fishes often of the same species and still oftener of the same genera as the British, whereas when they consist of limestone they contain shells similar to those of Devonshire, thus confirming, as Sir Roderick has pointed out, the contemporaneous origin which had been previously assigned to formations exhibiting two very distinct mineral types in different parts of Britain. (Murchison’s Siluria page 329.) The calcareous and the arenaceous rocks of Russia above alluded to alternate in such a manner as to leave no doubt of their having been deposited in different parts of the same great period.
DEVONIAN STRATA IN THE UNITED STATES AND CANADA.
(FIGURE 523. Psilophyton princeps, Dawson, Quarterly Geological Journal volume 15 1863; and Canada Survey 1863. Species characteristic of the whole Devonian series in North America.
a. Fruit; natural size.
b. Stem; natural size.
c. Scalariform tissue of the axis highly magnified.)
Between the Carboniferous and Silurian strata there intervenes, in the United States and Canada, a great series of formations referable to the Devonian group, comprising some strata of marine origin abounding in shells and corals, and others of shallow-water and littoral origin in which terrestrial plants abound. The fossils, both of the deep and shallow water strata, are very analogous to those of Europe, the species being in some cases the same. In Eastern Canada Sir W. Logan has pointed out that in the peninsula of Gaspe, south of the estuary of St. Lawrence, a mass of sandstone, conglomerate, and shale referable to this period occurs, rich in vegetable remains, together with some fish-spines. Far down in the sandstones of Gaspe, Dr. Dawson found, in 1869, an entire specimen of the genus Cephalaspis, a form so characteristic, as we have already seen, of the Scotch Lower Old Red Sandstone. Some of the sandstones are ripple-marked, and towards the upper part of the whole series a thin seam of coal has been observed, measuring, together with some associated carbonaceous shale, about three inches in thickness. It rests on an under-clay in which are the roots of Psilophyton (see Figure 523). At many other levels rootlets of this same plant have been shown by Principal Dawson to penetrate the clays, and to play the same part as do the rootlets of Stigmaria in the coal formation.
We had already learnt from the works of Goppert, Unger, and Bronn that the European plants of the Devonian epoch resemble generically, with few exceptions, those already known as Carboniferous; and Dr. Dawson, in 1859, enumerated 32 genera and 69 species which he had then obtained from the State of New York and Canada. A perusal of his catalogue (Quarterly Geological Journal volume 15 page 477 1859; also volume 18 page 296 1862.), comprising Coniferae, Sigillariae, Calamites, Asterophyllites, Lepidodendra, and ferns of the genera Cyclopteris, Neuropteris, Sphenopteris, and others, together with fruits, such as Cardiocarpum and Trigonocarpum, might dispose geologists to believe that they were presented with a list of Carboniferous fossils, the difference of the species from those of the coal-measures, and even a slight admixture of genera unknown in Europe, being naturally ascribed to geographical distribution and the distance of the New from the Old World. But fortunately the coal formation is fully developed on the other side of the Atlantic, and is singularly like that of Europe, both lithologically and in the species of its fossil plants. There is also the most unequivocal evidence of relative age afforded by superposition, for the Devonian strata in the United States are seen to crop out from beneath the Carboniferous on the borders of Pennsylvania and New York, where both formations are of great thickness.
The number of American Devonian plants has now been raised by Dr. Dawson to 120, to which we may add about 80 from the European flora of the same age, so that already the vegetation of this period is beginning to be nearly half as rich as that of the coal-measures which have been studied for so much longer a time and over so much wider an area. The Psilophyton above alluded to is believed by Dr. Dawson to be a lycopodiaceous plant, branching dichotomously (see P. princeps, Figure 523), with stems springing from a rhizome, which last has circular areoles, much resembling those of Stigmaria, and like it sending forth cylindrical rootlets. The extreme points of some of the branchlets are rolled up so as to resemble the croziers or circinate vernation of ferns; the leaves or bracts, a, supposed to belong to the same plant, are described by Dawson as having inclosed the fructification. The remains of Psilophyton princeps have been traced through all the members of the Devonian series in America, and Dr. Dawson has lately recognised it in specimens of Old Red Sandstone from the north of Scotland.
The monotonous character of the Carboniferous flora might be explained by imagining that we have only the vegetation handed down to us of one set of stations, consisting of wide swampy flats. But Dr. Dawson supposes that the geographical conditions under which the Devonian plants grew were more varied, and had more of an upland character. If so, the limitation of this more ancient flora, represented by so many genera and species, to the gymnospermous and cryptogamous orders, and the absence or extreme rarity of plants of higher grade, lead us naturally to speculate on the theory of progressive development, however difficult it may be to avail ourselves of this explanation, so long as we meet with even a few exceptional cases of what may seem to be monocotyledonous or dicotyledonous exogens.
DEVONIAN INSECTS OF CANADA.
The earliest known insects were brought to light in 1865 in the Devonian strata of St. John’s, New Brunswick, and are referred by Mr. Scudder to four species of Neuroptera. One of them is a gigantic Ephemera, and measured five inches in expanse of wing.
Like many other ancient animals, says Dr. Dawson, they show a remarkable union of characters now found in distinct orders of insects, or constitute what have been named “synthetic types.” Of this kind is a stridulating or musical apparatus like that of the cricket in an insect otherwise allied to the Neuroptera. This structure, as Dr. Dawson observes, if rightly interpreted by Mr. Scudder, introduces us to the sounds of the Devonian woods, bringing before our imagination the trill and hum of insect life that enlivened the solitudes of these strange old forests.
CHAPTER XXVI.
SILURIAN GROUP.
Classification of the Silurian Rocks. Ludlow Formation and Fossils.
Bone-bed of the Upper Ludlow.
Lower Ludlow Shales with Pentamerus. Oldest known Remains of fossil Fish.
Table of the progressive Discovery of Vertebrata in older Rocks. Wenlock Formation, Corals, Cystideans and Trilobites. Llandovery Group or Beds of Passage.
Lower Silurian Rocks.
Caradoc and Bala Beds.
Brachiopoda.
Trilobites.
Cystideae.
Graptolites.
Llandeilo Flags.
Arenig or Stiper-stones Group.
Foreign Silurian Equivalents in Europe. Silurian Strata of the United States.
Canadian Equivalents.
Amount of specific Agreement of Fossils with those of Europe.
CLASSIFICATION OF THE SILURIAN ROCKS.
We come next in descending order to that division of Primary or Palaeozoic rocks which immediately underlie the Devonian group or Old Red Sandstone. For these strata Sir Roderick Murchison first proposed the name of Silurian when he had studied and classified them in that part of Wales and some of the contiguous counties of England which once constituted the kingdom of the Silures, a tribe of ancient Britons. Table 26.1 will explain the two principal divisions, Upper and Lower, of the Silurian rocks, and the minor subdivisions usually adopted, comprehending all the strata originally embraced in the Silurian system by Sir Roderick Murchison. The formations below the Arenig or Stiper-stones group are treated of in the next chapter, when the “Primordial” or Cambrian group is described.
TABLE 26.1. SILURIAN ROCKS (THICKNESS GIVEN IN FEET).
UPPER SILURIAN ROCKS.
1. LUDLOW FORMATION:
a. Upper Ludlow beds: 780.
b. Lower Ludlow beds: 1,050.
2. WENLOCK FORMATION:
a. Wenlock limestone and shale and
b. Woolhope limestone and shale, and Denbighshire grits: above 4,000.
3. LLANDOVERY FORMATION (Beds of passage between Upper and Lower Silurian):
a. Upper Llandovery (May-Hill beds): 800. b. Lower Llandovery: 600-1,000.
LOWER SILURIAN ROCKS.
1. BALA AND CARADOC BEDS, including volcanic rocks: 12,000.
2. LLANDEILO FLAGS, including volcanic rocks: 4,500.
3. ARENIG OR STIPER-STONES GROUP, including volcanic rocks: above 10,000.
UPPER SILURIAN ROCKS.
1. LUDLOW FORMATION.
This member of the Upper Silurian group, as will be seen by Table 26.1, is of great thickness, and subdivided into two parts– the Upper Ludlow and the Lower Ludlow. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains; but out of more than 500 species found in the Ludlow formation as a whole, not more than five species per hundred are common to the overlying Devonian. The student may refer to the excellent tables given in the last edition of Sir R. Murchison’s Siluria for a list of the organic remains of all classes distributed through the different subdivisions of the Upper and Lower Silurian.
A. UPPER LUDLOW: DOWNTON SANDSTONE.
At the top of this subdivision there occur beds of fine-grained yellowish sandstone and hard reddish grits which were formerly referred by Sir R. Murchison to the Old Red Sandstone, under the name of “Tilestones.” In mineral character this group forms a transition from the Silurian to the Old Red Sandstone, the strata of both being conformable; but it is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the underlying Upper Ludlow rocks. Among these are Orthoceras bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, etc., with numerous defenses of fishes.
These beds, therefore, now generally called the “Downton Sandstone,” are classed as the newest member of the Upper Silurian. They are well seen at Downton Castle, near Ludlow, where they are quarried for building, and at Kington, in Herefordshire. In the latter place, as well as at Ludlow, crustaceans of the genera Pterygotus (for genus see Figure 504) and Eurypterus are met with.
BONE-BED OF THE UPPER LUDLOW.
At the base of the Downton sandstones there occurs a bone-bed which deserves especial notice as affording the most ancient example of fossil fish occurring in any considerable quantity. It usually consists of one or two thin layers of brown bony fragments near the junction of the Old Red Sandstone and the Ludlow rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where it is three or four inches thick. It has since been traced to a distance of 45 miles from that point into Gloucestershire and other counties, and is commonly not more than an inch thick, but varies to nearly a foot. Near Ludlow two bone- beds are observable, with 14 feet of intervening strata full of Upper Ludlow fossils. (Murchison’s Siluria page 140.) At that point immediately above the upper fish-bed numerous small globular bodies have been found, which were determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant, probably lycopodiaceous.
(FIGURE 524. Onchus tenuistriatus, Agassiz. Bone-bed. Upper Silurian. Ludlow.)
(FIGURE 525. Shagreen-scales of a placoid fish, Thelodus parvidens, Agassiz. Bone-bed, Upper Ludlow.)
(FIGURE 526. Plectrodus mirabilis, Agassiz. Bone-bed, Upper Ludlow.)
Most of the fish have been referred by Agassiz to his placoid order, some of them to the genus Onchus, to which the spine (Figure 524) and the minute scales (Figure 525) are supposed to belong. It has been suggested, however, that Onchus may be one of those Acanthodian fish referred by Agassiz to his Ganoid order, which are so characteristic of the base of the Old Red Sandstone in Forfarshire, although the species of the Old Red are all different from these of the Silurian beds now under consideration. The jaw and teeth of another predaceous genus (Figure 526) have also been detected, together with some specimens of Pteraspis Ludensis. As usual in bone-beds, the teeth and bones are, for the most part, fragmentary and rolled.
GREY SANDSTONE AND MUDSTONE, ETC.
(FIGURE 527. Orthis elegantula, Dalm. Var. Orbicularis, Sowerby. Upper Ludlow.)
(FIGURE 528. Rhynchonella navicula, Sowerby. Ludlow Beds.)
The next subdivision of the Upper Ludlow consists of grey calcareous sandstone, or very commonly a micaceous stone, decomposing into soft mud, and contains, besides the shells mentioned above, Lingula cornea, Orthis orbicularis, a round variety of O. elegantula, Modiolopsis platyphylla, Grammysia cingulata, all characteristic of the Upper Ludlow. The lowest or mud-stone beds contain Rhynchonella navicula (Figure 528), which is common to this bed and the Lower Ludlow. As usual in Palaeozoic strata older than the coal, the brachiopodous mollusca greatly outnumber the lamellibranchiate (see below); but the latter are by no means unrepresented. Among other genera, for example, we observe Avicula and Pterinea, Cardiola, Ctenodonta (sub-genus of Nucula), Orthonota, Modiolopsis, and Palaearca.
Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence of gradual deposition; and the same may be said of the accompanying fine argillaceous shales, which are of great thickness, and have been provincially named “mud-stones.” In some of these shales stems of crinoidea are found in an erect position, having evidently become fossil on the spots where they grew at the bottom of the sea. The facility with which these rocks, when exposed to the weather, are resolved into mud, proves that, notwithstanding their antiquity, they are nearly in the state in which they were first thrown down.
b. LOWER LUDLOW BEDS.
(FIGURE 529. Pentamerus Knightii, Sowerby. Aymestry. One-half natural size. a. View of both valves united.
b. Longitudinal section through both valves, showing the central plates or septa.)
The chief mass of this formation consists of a dark grey argillaceous shale with calcareous concretions, having a maximum thickness of 1000 feet. In some places, and especially at Aymestry, in Herefordshire, a subcrystalline and argillaceous limestone, sometimes 50 feet thick, overlies the shale. Sir R. Murchison therefore classes this Aymestry limestone as holding an intermediate position between the Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie, near Leintwardine, the Lower Ludlow shales, with their characteristic fossils, occur both above and below a similar limestone. This limestone around Aymestry and Sedgeley is distinguished by the abundance of Pentamerus Knightii, Sowerby (Figure 529), also found in the Lower Ludlow and Wenlock shale. This genus of brachiopoda was first found in Silurian strata, and is exclusively a palaeozoic form. The name was derived from pente, five, and meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five. The size of these septa is enormous compared with those of any other brachiopod shell; and they must nearly have divided the animal into two equal halves; but they are, nevertheless, of the same nature as the septa or plates which are found in the interior of Spirifera, Terebratula, and many other shells of this order. Messrs. Murchison and De Verneuil discovered this species dispersed in myriads through a white limestone of Upper Silurian age, on the banks of the Is, on the eastern flank of the Urals in Russia, and a similar species is frequent in Sweden.
(FIGURE 530. Lingula Lewisii, J. Sowb. Abberley Hills.)
(FIGURE 531. Rhynchonella (Terebratula) Wilsoni, Sowerby. Aymestry.)
(FIGURE 532. Atrypa reticularis, Linn. (Terebratula affinis, Min. Con.) Aymestry.
a. Upper valve.
b. Lower valve.
c. Anterior margin of the valves.)
Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii (Figure 530); second, Rhynchonella Wilsoni, Sowerby. (Figure 531), which is also common to the Lower Ludlow and Wenlock limestone; third, Atrypa reticularis, Linn. (Figure 532), which has a very wide range, being found in every part of the Upper Silurian system, and even ranging up into the Middle Devonian series.
The Aymestry Limestone contains many shells, especially brachiopoda, corals, trilobites, and other fossils, amounting on the whole to 74 species, all except three or four being common to the beds either above or below.
(FIGURE 533. Phragmoceras ventricosum, J. Sowerby. (Orthoceras ventricosum, Stein.) Aymestry; one-quarter natural size.)
(FIGURE 534. Lituites (Trochoceras) giganteus, J. Sowerby. Near Ludlow; also in the Aymestry and Wenlock Limestones; 1/4 natural size.)
(FIGURE 535. Fragment of Orthoceras Ludense, J. Sowerby. Leintwardine, Shropshire.)
The Lower Ludlow Shale contains, among other fossils, many large cephalopoda not known in newer rocks, as the Phragmoceras of Broderip, and the Lituites of Breynius (see Figures 533, 534). The latter is partly straight and partly convoluted in a very flat spire. The Orthoceras Ludense (Figure 535), as well as the cephalopod last mentioned, occurs in this member of the species.
A species of Graptolite, G. priodon, Bronn (Figure 545), occurs plentifully in the Lower Ludlow. This fossil, referred, though somewhat doubtfully, to a form of hydrozoid or sertularian polyp, has not yet been met with in strata above the Silurian.
Star-fish, as Sir R. Murchison points out, are by no means rare in the Lower Ludlow rock. These fossils, of which six extinct genera are now known in the Ludlow series, represented by 18 species, remind us of various living forms now found in our British seas, both of the families Asteriadae and Ophiuridae.
OLDEST KNOWN FOSSIL FISH.
Until 1859 there was no example of a fossil fish older than the bone-bed of the Upper Ludlow, but in that year a specimen of Pteraspis was found at Church Hill, near Leintwardine, in Shropshire, by Mr. J.E. Lee of Caerleon, F.G.S., in shale below the Aymestry limestone, associated with fossil shells of the Lower Ludlow formation– shells which differ considerably from those characterising the Upper Ludlow already described. This discovery is of no small interest as bearing on the theory of progressive development, because, according to Professor Huxley, the genus Pteraspis is allied to the sturgeon, and therefore by no means of low grade in the piscine class.
It is a fact well worthy of notice that no remains of vertebrata have yet been met with in any strata older than the Lower Ludlow.
When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, and other fossils already obtained from more ancient Silurian formations, Upper, Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer in the series were ever studied with equal diligence, and over so vast an area, without yielding a single ichthyolite. Yet we must hesitate before we accept, even on such evidence, so sweeping a conclusion, as that the globe, for ages after it was inhabited by all the great classes of invertebrata, remained wholly untenanted by vertebrate animals.
TABLE 26.2. DATES OF THE DISCOVERY OF DIFFERENT CLASSES OF FOSSIL VERTEBRATA; SHOWING THE GRADUAL PROGRESS MADE IN TRACING THEM TO ROCKS OF HIGHER ANTIQUITY.
COLUMN 1: YEAR.
COLUMN 2: FORMATIONS.
COLUMN 3: GEOGRAPHICAL LOCALITIES.
MAMMALIA:
1798: Upper Eocene: Paris (Gypsum of Montmartre). (George Cuvier, Bulletin Soc. Philom. 20.)
1818: Lower Oolite: Stonesfield. (In 1818, Cuvier, visiting the Museum of Oxford, decided on the mammalian character of a jaw from Stonesfield. See also above Chapter 19.)
1847: Upper Trias: Stuttgart. (Professor Plieninger. See above Chapter 21.)
AVES:
1782: Upper Eocene: Paris (Gypsum of Montmartre). (Cuvier, Ossemens Foss. Art. “Oiseaux.”)
1839: Lower Eocene: Isle of Sheppey (London Clay). (Professor Owen Geological Transactions second series volume 6 page 203 1839.)
1854: Lower Eocene: Woolwich Beds. (Upper part of the Woolwich beds. Prestwich Quarterly Geological Journal volume 10 page 157.)
1855: Lower Eocene: Meudon (Plastic Clay). (Gastornis Parisiensis. Owen Quarterly Geological Journal volume 12 page 204 1856.)
1858: Chloritic Series, or Upper Greensand: Cambridge. (Coprolitic bed, in the Upper Greensand. See above Chapter 17.)
1863: Upper Oolite: Solenhofen. (The Archaeopteryx macrura, Owen. See above Chapter 19.)
REPTILIA:
1710: Permian (or Zechstein): Thuringia. (The fossil monitor of Thuringia (Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810. (Miscel. Berlin.))
1844: Carboniferous: Saarbruck, near Treves. (See Chapter 23.)
PISCES:
1709: Permian (or Kupferschiefer): Thuringia. (Memorabilia Saxoniae Subterr. Leipsic 1709.)
1793: Carboniferous (Mountain Limestone): Glasgow. (History of Rutherglen by David Ure, 1793.)
1828: Devonian: Caithness. (Sedgwick and Murchison Geological Transactions second series volume 3 page 141 1828.)
1840: Upper Ludlow: Ludlow. (Sir R. Murchison. See Chapter 26.)
1859: Lower Ludlow: Leintwardine. (See Chapter 26.)
Obs.– The evidence derived from foot-prints, though often to be relied on, is omitted in the above table, as being less exact than that founded on bones and teeth.
In Table 26.2 a few dates are set before the reader of the discovery of different classes of animals in ancient rocks, to enable him to perceive at a glance how gradual has been our progress in tracing back the signs of vertebrata to formations of high antiquity. Such facts may be useful in warning us not to assume too hastily that the point which our retrospect may have reached at the present moment can be regarded as fixing the date of the first introduction of any one class of beings upon the earth.
2. WENLOCK FORMATION.
We next come to the Wenlock formation, which has been divided into Wenlock limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.
a. WENLOCK LIMESTONE.
This limestone, otherwise well known to collectors by the name of the Dudley Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. This ridgy prominence is due to the solidity of the rock, and to the softness of the shales above and below it. Near Wenlock it consists of thick masses of grey subcrystalline limestone, replete with corals, encrinites, and trilobites. It is essentially of a concretionary nature; and the concretions, termed “ball-stones” in Shropshire, are often enormous, even 80 feet in diameter. They are of pure carbonate of lime, the surrounding rock being more or less argillaceous (Murchison’s Siluria chapter 6.) Sometimes in the Malvern Hills this limestone, according to Professor Phillips, is oolitic.
(FIGURE 536. Halysites catenularius, Linn. sp. Upper and Lower Silurian.)
(FIGURE 537. Favosites Gothlandica, Lam. Dudley. a. Portion of a large mass; less than the natural size. b. Magnified portion, to show the pores and the partitions in the tubes.)
(FIGURE 538. Omphyma turbinatum, Linn. Sp. (Cyathophyllum, Goldfuss) Wenlock Limestone, Shropshire.)
Among the corals, in which this formation is so rich, 53 species being known, the “chain-coral,” Halysites catenularius (Figure 536), may be pointed out as one very easily recognised, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to near the bottom of the Llandeilo rocks. Another coral, the Favosites Gothlandica (Figure 537), is also met with in profusion in large hemispherical masses, which break up into columnar and prismatic fragments, like that here figured (Figure 537, b). Another common form in the Wenlock limestone is the Omphyma turbinatum (Figure 538), which, like many of its modern companions, reminds us of some cup-corals; but all the Silurian genera belong to the palaeozoic type before mentioned (Chapter 24), exhibiting the quadripartite arrangement of the septalamellae within the cup.
(FIGURE 539. Pseudocrintes bifasciatus, Pearce. Wenlock Limestone, Dudley.)
Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus see Figures 478, 479) contribute their calcareous stems, arms, and cups towards the composition of the Wenlock limestone. Of Cystideans there are a few very remarkable forms, most of them peculiar to the Upper Silurian formation, as, for example, the Pseudocrinites, which was furnished with pinnated fixed arms, as represented in Figure 539. (E. Forbes Mem. Geological Survey volume 2 page 496.)
(FIGURE 540. Strophomena (Leptaena) depressa, Sowerby. Wenlock and Ludlow Rocks.)
The Brachiopoda are, many of them, of the same species as those of the Aymestry limestone; as, for example, Atrypa reticularis (Figure 532), and Strophomena depressa (Figure 540); but the latter species ranges also from the Ludlow rocks, through the Wenlock shale, to the Caradoc Sandstone.
(FIGURE 541. Calymene Blumenbachii, Brong. Ludlow, Wenlock, and Bala beds.)
(FIGURE 542. Phacops (Asaphus) caudatus, Brong. Wenlock and Ludlow Rocks.)
(FIGURE 543. Sphaerexochus mirus, Beyrich; coiled up. Wenlock Limestone, Dudley; also found in Ohio, North America.)
(FIGURE 544. Homalonotus delphinocephalus, Konig. Wenlock Limestone, Dudley Castle.)
The crustaceans are represented almost exclusively by Trilobites, which are very conspicuous, 22 being peculiar. The Calymene Blumenbachii (Figure 541), called the “Dudley Trilobite,” was known to collectors long before its true place in the animal kingdom was ascertained. It is often found coiled up like the common Oniscus or wood-louse, and this is so usual a circumstance among certain genera of trilobites as to lead us to conclude that they must have habitually resorted to this mode of protecting themselves when alarmed. The other common species is the Phacops caudatus (Asaphus caudatus), Brong. (see Figure 542), and this is conspicuous for its large size and flattened form. Sphaerexochus mirus (Figure 543) is almost a globe when rolled up, the forehead or glabellum of this species being extremely inflated. The Homalonotus, a form of Trilobite in which the tripartite division of the dorsal crust is almost lost (see Figure 544), is very characteristic of this division of the Silurian series.
WENLOCK SHALE.
(FIGURE 545. Graptolithus priodon, Bronn. Ludlow and Wenlock shales.)
The Wenlock Shale, observes Sir R. Murchison, is infinitely the largest and most persistent member of the Wenlock formation, for the limestone often thins out and disappears. The shale, like the Lower Ludlow, often contains elliptical concretions of impure earthy limestone. In the Malvern district it is a mass of finely levigated argillaceous matter, attaining, according to Professor Phillips, a thickness of 640 feet, but it is sometimes more than 1000 feet thick in Wales, and is worked for flag-stones and slates. The prevailing fossils, besides corals and trilobites, and some crinoids, are several small species of Orthis, Cardiola, and numerous thin-shelled species of Orthoceratites.
About six species of Graptolite, a peculiar group of sertularian fossils before alluded to as being confined to Silurian rocks, occur in this shale. Of fossils of this genus, which is very characteristic of the Lower Silurian, I shall again speak in the sequel.
b. WOOLHOPE BEDS.
Though not always recognised as a separate subdivision of the Wenlock, the Woolhope beds, which underlie the Wenlock shale, are of great importance. Usually they occur as massive or nodular limestones, underlaid by a fine shale or flag-stone; and in other cases, as in the noted Denbighshire sandstones, as a coarse grit of very great thickness. This grit forms mountain ranges through North and South Wales, and is generally marked by the great sterility of the soil where it occurs. It contains the usual Wenlock fossils, but with the addition of some common in the uppermost Ludlow rock, such as Chonetes lata and Bellerophon trilobatus. The chief fossils of the Woolhope limestone are Illaenus Barriensis, Homalonotus delphinocephalus (Figure 544), Strophomena imbrex, and Rhynchonella Wilsoni (Figure 531). The latter attains in the Woolhope beds an unusual size for the species, the specimens being sometimes twice as large as those found in the Wenlock limestone.
In some places below the Wenlock formation there are shales of a pale or purple colour, which near Tarannon attain a thickness of about 1000 feet; they can be traced through Radnor and Montgomery to North Wales, according to Messrs. Jukes and Aveline. By the latter geologist they have been identified with certain shales above the May-Hill Sandstone, near Llandovery, but, owing to the extreme scarcity of fossils, their exact position remains doubtful.
3. LLANDOVERY GROUP– BEDS OF PASSAGE.
We now come to beds respecting the classification of which there has been much difference of opinion, and which in fact must be considered as beds of passage between Upper and Lower Silurian. I formerly adopted the plan of those who class them as Middle Silurian, but they are scarcely entitled to this distinction, since after about 1400 Silurian species have been compared the number peculiar to the group in question only gives them an importance equal to such minor subdivisions as the Ludlow or Bala groups. I therefore prefer to regard them as the base of the Upper Silurian, to which group they are linked by more than twice as many species as to the Lower Silurian. By this arrangement the line of demarkation between the two great divisions, though confessedly arbitrary, is less so than by any other. They are called Llandovery Rocks, from a town in South Wales, in the neighbourhood of which they are well developed, and where, especially at a hill called Noeth Grug, in spite of several faults, their relations to one another can be clearly seen.
a. UPPER LLANDOVERY OR MAY-HILL SANDSTONE.
(FIGURE 546. Pentamerus oblongus, Sowerby. Upper and Lower Llandovery beds. a, b. Views of the shell itself, from figures in Murchison’s Sil. Syst. c. Cast with portion of shell remaining, and with the hollow of the central septum filled with spar.
d. Internal cast of a valve, the space once occupied by the septum being represented by a hollow in which is seen a cast of the chamber within the septum.)
(FIGURE 547. Stricklandinia (Pentamerus) lirata, Sowerby.)
The May-Hill group, which has also been named “Upper Llandovery,” by Sir R. Murchison, ranges from the west of the Longmynd to Builth, Llandovery, and Llandeilo, and to the sea in Marlow’s Bay, where it is seen in the cliffs. It consists of brownish and yellow sandstones with calcareous nodules, having sometimes a conglomerate at the base derived from the waste of the Lower Silurian rocks. These May-Hill beds were formerly supposed to be part of the Caradoc formation, but their true position was determined by Professor Sedgwick to be at the base of the Upper Silurian proper. (Quarterly Geological Journal volume 4 page 215 1853.) The more calcareous portions of the rock have been called the Pentamerus limestone, because Pentamerus oblongus (Figure 546) is very abundant in them. It is usually accompanied by P. (Stricklandinia) lirata (Figure 547); both forms have a wide geographical range, being also met with in the same part of the Silurian series in Russia and the United States.
About 228 species of fossils are known in the May-Hill division, more than half of which are Wenlock species. They consist of trilobites of the genera Illaenus and Calymene; Brachiopods of the genera Orthis, Atrypa, Leptaena, Pentamerus, Strophomena, and others; Gasteropods of the genera Turbo, Murchisonia (for genus, see Figure 567), and Bellerophon; and Pteropods of the genus Conularia. The Brachiopods, of which there are 66 species, are almost all Upper Silurian.
(FIGURE 548. Tentaculites annulatus, Schlot. Interior casts in sandstone. Upper Llandovery, Eastnor Park, near Malvern. Natural size and magnified.)
Among the fossils of the May-Hill shelly sandstone at Malvern, Tentaculites annulatus (Figure 548), an annelid, probably allied to Serpula, is found.
LOWER LLANDOVERY ROCKS.
Below the May-Hill Group are the Lower Llandovery Rocks, which consist chiefly of hard slaty rocks, and beds of conglomerate from 600 to 1000 feet in thickness. The fossils, which are somewhat rare in the lower beds, consist of 128 known species, only eleven of which are peculiar, 83 being common to the May-Hill group above, and 93 common to the rocks below. Stricklandinia (Pentamerus) levis, which is common in the Lower Llandovery, becomes rare in the Upper, while Pentamerus oblongus (Figure 546), which is the characteristic shell of the Upper Llandovery, occurs but seldom in the Lower.
LOWER SILURIAN ROCKS.
The Lower Silurian has been divided into, first, the Bala Group; secondly, the Llandeilo flags; and, thirdly, the Arenig or Lower Llandeilo formation.
BALA AND CARADOC BEDS.
(FIGURE 549. Orthis tricenaria, Conrad. New York; Canada. 1/2 natural size.)
(FIGURE 550. Orthis vespertilio, Sowerby. Shropshire, N. and S. Wales. One-half natural size.)
(FIGURE 551. Orthis (Strophomena) grandis, Sowerby. Two-thirds natural size. Caradoc Beds, Horderley, Shropshire, and Coniston, Lancashire.)
The Caradoc sandstone was originally so named by Sir R.I. Murchison from the mountain called Caer Caradoc, in Shropshire; it consists of shelly sandstones of great thickness, and sometimes containing much calcareous matter. The rock is frequently laden with the beautiful trilobite called by Murchison Trinucleus Caractaci (see Figure 553), which ranges from the base to the summit of the formation, usually accompanied by Strophomena grandis (see Figure 551), and Orthis vespertilio (Figure 550), with many other fossils.
BRACHIOPODA.
Nothing is more remarkable in these beds, and in the Silurian strata generally of all countries, than the preponderance of brachiopoda over other forms of mollusca. Their proportional numbers can by no means be explained by supposing them to have inhabited seas of great depth, for the contrast between the palaeozoic and the present state of things has not been essentially altered by the late discoveries made in our deep-sea dredgings. We find the living brachiopoda so rare as to form about one forty-fourth of the whole bivalve fauna, whereas in the Lower Silurian rocks of which we are now about to treat, and where the brachiopoda reach their maximum, they are represented by more than twice as many species as the Lamellibranchiate bivalves.
There may, indeed, be said to be a continued decrease of the proportional number of this lower tribe of mollusca as we proceed from older to newer rocks. In the British Devonian, for example, the Brachiopoda number 99, the Lamellibranchiata 58; while in the Carboniferous their proportions are more than reversed, the Lamellibranchiata numbering 334 species, and the Brachiopoda only 157. In the Secondary or Cainozoic formations the preponderance of the higher grade of bivalves becomes more and more marked, till in the tertiary strata it approaches that observed in the living creation.
While on this subject, it may be useful to the student to know that a Brachiopod differs from ordinary bivalves, mussels, cockles, etc., in being always equal- sided and never quite equi-valved; the form of each valve being symmetrical, it may be divided into two equal parts by a line drawn from the apex to the centre of the margin.
TRILOBITES.
In the Bala and Caradoc beds the trilobites reach their maximum, being represented by 111 species referred to 23 genera.
(FIGURE 552. Young individuals of Trinucleus concentricus (T. ornatus, Barr.). a. Youngest state. Natural size and magnified; the body rings not at all developed.
b. A little older. One thorax joint. c. Still more advanced. Three thorax joints. The fourth, fifth, and sixth segments are successively produced, probably each time the animal moulted its crust.)
(FIGURE 553. Trinucleus concentricus, Eaton. Syn. T. Caractaci, Murch. Ireland; Wales; Shropshire; North America; Bohemia.)
Burmeister, in his work on the organisation of trilobites, supposes that they swam at the surface of the water in the open sea and near coasts, feeding on smaller marine animals, and to have had the power of rolling themselves into a ball as a defence against injury. He was also of opinion that they underwent various transformations analogous to those of living crustaceans. M. Barrande, author of an admirable work on the Silurian rocks of Bohemia, confirms the doctrine of their metamorphosis, having traced more than twenty species through different stages of growth from the young state just after its escape from the egg to the adult form. He has followed some of them from a point in which they show no eyes, no joints, or body rings, and no distinct tail, up to the complete form with the full number of segments. This change is brought about before the animal has attained a tenth part of its full dimensions, and hence such minute and delicate specimens are rarely met with. Some of his figures of the metamorphoses of the common Trinucleus are copied in Figures 552 and 553. It was not till 1870 that Mr. Billings was enabled, by means of a specimen found in Canada, to prove that the trilobite was provided with eight legs.
(FIGURE 554. Palaeaster asperimus, Salt. Caradoc, Welshpool.)
(FIGURE 555. Echinosphaeronites balticus, Eichwald. (Of the family Cystideae.) a. Mouth.
b. Point of attachment of stem. Lower Silurian S. and N. Wales.)
It has been ascertained that a great thickness of slaty and crystalline rocks of South Wales, as well as those of Snowdon and Bala, in North Wales, which were first supposed to be of older date than the Silurian sandstones and mudstones of Shropshire, are in fact identical in age, and contain the same organic remains. At Bala, in Merionethshire, a limestone rich in fossils occurs, in which two genera of star-fish, Protaster and Palaeaster, are found; the fossil specimen of the latter (Figure 554) being almost as uncompressed as if found just washed up on the sea-beach. Besides the star-fish there occur abundance of those peculiar bodies called Cystideae. They are the Sphaeronites of old authors, and were considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Echinosphaeronite here represented (Figure 555) is characteristic of the Caradoc beds in Wales, and of their equivalents in Sweden and Russia.
With it have been found several other genera of the same family, such as Sphaeronites, Hemicosmites, etc. Among the mollusca are Pteropods of the genus Conularia of large size (for genus, see Figure 518). About eleven species of Graptolite are reckoned as belonging to this formation; they are chiefly found in peculiar localities where black mud abounded. The formation, when traced into South Wales and Ireland, assumes a greatly altered mineral aspect, but still retains its characteristic fossils. The known fauna of the Bala group comprises 565 species, 352 of which are peculiar, and 93, as before stated, are common to the overlying Llandovery rocks. It is worthy of remark that, when it occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which distinguish it from the Llandeilo beds are still observable. The formation generally appears to be of shallow-water origin, and in that respect is contrasted with the group next to be described. Professor Ramsay estimates the thickness of the Bala Beds, including the contemporaneous volcanic rocks, stratified and unstratified, as being from 10,000 to 12,000 feet.
LLANDEILO FLAGS.
(FIGURE 556. Didymograpsus (Graptolites) Murchisonii, Beck. Llandeilo flags, Wales.)
The Lower Silurian strata were originally divided by Sir R. Murchison into the upper group already described, under the name of Caradoc Sandstone, and a lower one, called, from a town in Carmarthenshire, the Llandeilo flags. The last mentioned strata consist of dark-coloured micaceous flags, frequently calcareous, with a great thickness of shales, generally black, below them. The same beds are also seen at Builth, in Radnorshire, where they are interstratified with volcanic matter.
(FIGURE 557. Diplograpsus pristis, Hisinger. Llandeilo beds, Waterford.)
(FIGURE 558. Rastrites peregrinus, Barrande. Scotland; Bohemia; Saxony. Llandeilo flags.)
(FIGURE 559. Diplograpsus folium, Hisinger. Dumfriesshire; Sweden. Llandeilo flags.)
A still lower part of the Llandeilo rocks consists of a black carbonaceous slate of great thickness, frequently containing sulphate of alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It has been conjectured that this carbonaceous matter may be due in great measure to large quantities of imbedded animal remains, for the number of Graptolites included in these slates was certainly very great. In Great Britain eleven genera and about 40 species of Graptolites occur in the Llandeilo flags and underlying Arenig beds. The double Graptolites, or those with two rows of cells, such as Diplograpsus (Figure 557), are conspicuous.
The brachiopoda of the Llandeilo flags, which number 47 species, are in the main the same as those of the Caradoc Sandstone, but the other mollusca are in great part of different species.
(FIGURE 560. Orthoceras duplex, Wahlenberg. Russia and Sweden. (From Murchison’s Siluria.))
(FIGURE 561. Asaphus tyrannus, Murchison. Llandeilo; Bishop’s Castle; etc.)
(FIGURE 562. Ogygia Buchii, Burm. Syn. Asaphus Buchii, Brongn. Builth, Radnorshire; Llandeilo, Carmarthenshire.)
In Europe generally, as, for example, in Sweden and Russia, no shells are so characteristic of this formation as Orthoceratites, usually of great size, and with a wide siphuncle placed on one side instead of being central (see Figure 560). Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same beds also are found Bellerophon (see Figure 488) and some Pteropod shells (Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate bivalves of large size. The Crustaceans were plentifully represented by the Trilobites, which appear to have swarmed in the Silurian seas just as crabs and shrimps do in our own; no less than 263 species have been found in the British Silurian fauna. The genera Asaphus (Figure 561), Ogygia (Figure 562), and Trinucleus (Figures 552 and 553) form a marked feature of the rich and varied Trilobitic fauna of this age.
Beneath the black slates above described of the Llandeilo formation, Graptolites are still found in great variety and abundance, and the characteristic genera of shells and trilobites of the Lower Silurian rocks are still traceable downward, in Shropshire, Cumberland, and North and South Wales, through a vast depth of shaly beds, in some districts interstratified with trappean formations of contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed of such materials as are ejected from craters and deposited immediately on the bed of the ocean, or washed into it from the land. According to Professor Ramsay, their thickness is about 3300 feet in North Wales, including those of the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, and, according to the same authority, of an aggregate thickness of 2500 feet.
ARENIG OR STIPER-STONES GROUP (LOWER LLANDEILO OF MURCHISON).
(FIGURE 563. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones. a. Parting between the beds, or planes of bedding.)
(FIGURE 564. Didymograpsus geminus, Hisinger, sp. Sweden.)
Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. Originally these Stiper-Stones were only known as arenaceous quartzose strata in which no organic remains were conspicuous, except the tubular burrows of annelids (see Figure 563, Arenicolites linearis), which are remarkably common in the Lowest Silurian in Shropshire, and in the State of New York, in America. They have already been alluded to as occurring by thousands in the Silurian strata unconformably overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire (Figure 82). I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait. When the term Silurian was given by Sir R. Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the base of the Silurian system, but no fossil fauna had then been obtained, such as could alone enable the geologist to draw a line between this member of the series and the Llandeilo flags above, or a vast thickness of rock below, which was seen to form the Longmynd hills, and was called “unfossiliferous graywacke.” Professor Sedgwick had described, in 1843, strata now ascertained to be of the same age as largely developed in the Arenig mountain, in Merionethshire; and the Skiddaw slates in the Lake-District of Cumberland, studied by the same author, were of corresponding date, though the number of fossils was, in both cases, too few for the determination of their true chronological relations. The subsequent researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir R.I. Murchison and the Government surveyors in Shropshire, have increased the species to more than sixty. These were examined by Mr. Salter, and shown in the third edition of “Siluria” (page 52, 1859) to be quite distinct from the fossils of the overlying Llandeilo flags. Among these the Obolella plumbea, Aeglina binodosa, Ogygia Selwynii, and Didymograpsus geminus (Figure 564), and D. Hirundo, are characteristic.
But, although the species are distinct, the genera are the same as those which characterise the Silurian rocks above, and none of the characteristic primordial or Cambrian forms, presently to be mentioned, are intermixed. The same may be said of a set of beds underlying the Arenig rocks at Ramsay Island and other places in the neighbourhood of St. David’s. These beds, which have only lately become known to us through the labours of Dr. Hicks (Transactions of the British Association 1866. Proceedings of the Liverpool Geological Society 1869.), present already twenty new species, the greater part of them allied generically to the Arenig rocks. This Arenig group may therefore be conveniently regarded as the base of the great Silurian system, a system which, by the thickness of its strata and the changes in animal life of which it contains the record, is more than equal in value to the Devonian, or Carboniferous, or other principal divisions, whether of primary or secondary date.
It would be unsafe to rely on the mere thickness of the strata, considered apart from the great fluctuations in organic life which took place between the era of the Llandeilo and that of the Ludlow formation, especially as the enormous pile of Silurian rocks observed in Great Britain (in Wales more particularly) is derived in great part from igneous action, and is not confined to the ordinary deposition of sediment from rivers or the waste of cliffs.
In volcanic archipelagoes, such as the Canaries, we see the most active of all known causes, aqueous and igneous, simultaneously at work to produce great results in a comparatively moderate lapse of time. The outpouring of repeated streams of lava– the showering down upon land and sea of volcanic ashes– the sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles and sand, by rivers and torrents descending steeply inclined channels– the undermining and eating away of long lines of sea-cliff exposed to the swell of a deep and open ocean– these operations combine to produce a considerable volume of superimposed matter, without there being time for any extensive change of species. Nevertheless, there would seem to be a limit to the thickness of stony masses formed even under such favourable circumstances, for the analogy of tertiary volcanic regions lends no countenance to the notion that sedimentary and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales, could originate while one and the same fauna should continue to people the earth. If, then, we allow that about 25,000 feet of matter may be ascribed to one system, such as the Silurian, as above described, we may be prepared to discover in the next series of subjacent rocks a distinct assemblage of species, or even in great part of genera, of organic remains. Such appears to be the fact, and I shall therefore conclude with the Arenig beds my enumeration of the Silurian formations in Great Britain, and proceed to say something of their foreign equivalents, before treating of rocks older than the Silurian.
SILURIAN STRATA OF THE CONTINENT OF EUROPE.
When we turn to the continent of Europe, we discover the same ancient series occupying a wide area, but in no region as yet has it been observed to attain great thickness. Thus, in Norway and Sweden, the total thickness of strata of Silurian age is considerably less than 1000 feet, although the representatives both of the Upper and Lower Silurian of England are not wanting there. In Russia the Silurian strata, so far as they are yet known, seem to be even of smaller vertical dimensions than in Scandinavia, and they appear to consist chiefly of the Llandovery group, or of a limestone containing Pentamerus oblongus, below which are strata with fossils corresponding to those of the Llandeilo beds of England. The lowest rock with organic remains yet discovered is “the Ungulite or Obolus grit” of St. Petersburg, probably coeval with the Llandeilo flags of Wales.
(Figures 565 and 566. Shells of the lowest known Fossiliferous Beds in Russia.
(FIGURE 565. Siphonotreta unguiculata, Eichwald. From the Lowest Silurian Sandstone, “Obolus grits,” of St. Petersburg. a. Outside of perforated valve.
b. Interior of same, showing the termination of the foramen within. (Davidson.))
(FIGURE 566. Obolus Apollinis, Eichwald. From the same locality. a. Interior of the larger or ventral valve. b. Exterior of the upper (dorsal) valve. (Davidson, “Palaeontographic Monograph.”)))
The shales and grits near St. Petersburg, above alluded to, contain green grains in their sandy layers, and are in a singularly unaltered state, taking into account their high antiquity. The prevailing Brachiopods consist of the Obolus or Ungulite of Pander, and a Siphonotreta (Figures 565, 566). Notwithstanding the antiquity of this Russian formation, it should be stated that both of these genera of brachiopods have been also found in the Upper Silurian of England, i.e. In the Wenlock limestone.
Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg announced in 1854 his discovery of remains of foraminifera. These are casts of the cells; and among five or six forms three are considered by him as referable to existing genera (e.g., Textularia, Rotalia, and Guttulina).
SILURIAN STRATA OF THE UNITED STATES.
Table 26.3. SUBDIVISIONS OF THE SILURIAN STRATA OF NEW YORK. (Strata below the Oriskany sandstone or base of the Devonian.)
COLUMN 1: NEW YORK NAMES.
COLUMN 2: BRITISH EQUIVALENTS.
1. Upper Pentamerus Limestone: Upper Silurian (or Ludlow and Wenlock formations).
2. Encrinal Limestone: Upper Silurian (or Ludlow and Wenlock formations).
3. Delthyris Shaly Limestone: Upper Silurian (or Ludlow and Wenlock formations).
4. Pentamerus and Tentaculite Limestones: Upper Silurian (or Ludlow and Wenlock formations).
5. Water Lime Group: Upper Silurian (or Ludlow and Wenlock formations).
6. Onondaga Salt Group: Upper Silurian (or Ludlow and Wenlock formations).
7. Niagara Group: Upper Silurian (or Ludlow and Wenlock formations).
8. Clinton Group: Beds of Passage, Llandovery Group.
9. Medina Sandstone: Beds of Passage, Llandovery Group.
10. Oneida Conglomerate: Beds of Passage, Llandovery Group.
11. Gray Sandstone: Beds of Passage, Llandovery Group.
12. Hudson River Group: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).
13. Trenton Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).
14. Black-River Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).
15. Bird’s-eye Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).
16. Chazy Limestone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).
17. Calciferous Sandstone: Lower Silurian (or Caradoc and Bala, Llandeilo and Arenig Formations).
The Silurian formations can be advantageously studied in the States of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are often found, as in Russia, nearly in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. In the State of New York, where the succession of the beds and their fossils have been most carefully worked out by the Government surveyors, the subdivisions given in the first column of Table 26.3 have been adopted.
In the second column of the same table I have added the supposed British equivalents. All Palaeontologists, European and American, such as MM. De Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered upon this comparison, admit that there is a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downward from the highest to the lowest beds; but it is impossible to parallel each minor subdivision.
That the Niagara Limestone, over which the river of that name is precipitated at the great cataract, together with its underlying shales, corresponds to the Wenlock limestone and shale of England there can be no doubt. Among the species common to this formation in America and Europe are Calymene Blumenbachii, Homalonotus delphinocephalus (Figure 544), with several other trilobites; Rhynchonella Wilsoni, Figure 531, and Retzia cuneata; Orthis elegantula, Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the cephalopodous shells; and Favosites gothlandica, with other large corals.
The Clinton Group, containing Pentamerus oblongus and Stricklandinia, and related more nearly by its fossil species with the beds above than with those below, is the equivalent of the Llandovery Group or beds of passage.
(FIGURE 567. Murchisonia gracilis, Hall. A fossil characteristic of the Trenton Limestone. The genus is common in Lower Silurian rocks.)
The Hudson River Group, and the Trenton Limestone, agree palaeontologically with the Caradoc or Bala group, containing in common with them several species of trilobites, such as Asaphus (Isotelus) gigas, Trinucleus concentricus (Figure 553); and various shells, such as Orthis striatula, Orthis biforata (or O. lynx), O. porcata (O. occidentalis of Hall), and Bellerophon bilobatus. In the Trenton limestone occurs Murchisonia gracilis, Figure 567, a fossil also common to the Llandeilo beds in England.
Mr. D. Sharpe, in his report on the mollusca collected by me from these strata in North America (Quarterly Geological Journal volume 4.), has concluded that the number of species common to the Silurian rocks on both sides of the Atlantic is between 30 and 40 per cent; a result which, although no doubt liable to future modification, when a larger comparison shall have been made, proves, nevertheless, that many of the species had a wide geographical range. It seems that comparatively few of the gasteropods and lamellibranchiate bivalves of North America can be identified specifically with European fossils, while no less than two-fifths of the brachiopoda, of which my collection chiefly consisted, are the same. In explanation of these facts, it is suggested that most of the recent brachiopoda (especially the orthidiform ones) are inhabitants of deep water, and that they may have had a wider geographical range than shells living near shore. The predominance of bivalve mollusca of this peculiar class has caused the Silurian period to be sometimes styled “the age of brachiopods.”
In Canada, as in the State of New York, the Potsdam Sandstone underlies the above-mentioned calcareous rocks, but contains a different suite of fossils, as will be hereafter explained. In parts of the globe still more remote from Europe the Silurian strata have also been recognised, as in South America, Australia, and India. In all these regions the facies of the fauna, or the types of organic life, enable us to recognise the contemporaneous origin of the rocks; but the fossil species are distinct, showing that the old notion of a universal diffusion throughout the “primaeval seas” of one uniform specific fauna was quite unfounded, geographical provinces having evidently existed in the oldest as in the most modern times.
CHAPTER XXVII.
CAMBRIAN AND LAURENTIAN GROUPS.
Classification of the Cambrian Group, and its Equivalent in Bohemia. Upper Cambrian Rocks.
Tremadoc Slates and their Fossils.
Lingula Flags.
Lower Cambrian Rocks.
Menevian Beds.
Longmynd Group.
Harlech Grits with large Trilobites. Llanberis Slates.
Cambrian Rocks of Bohemia.
Primordial Zone of Barrande.
Metamorphosis of Trilobites.
Cambrian Rocks of Sweden and Norway. Cambrian Rocks of the United States and Canada. Potsdam Sandstone.
Huronian Series.
Laurentian Group, upper and lower.
Eozoon Canadense, oldest known Fossil. Fundamental Gneiss of Scotland.
CAMBRIAN GROUP.
The characters of the Upper and Lower Silurian rocks were established so fully, both on stratigraphical and palaeontological data, by Sir Roderick Murchison after five years’ labour, in 1839, when his “Silurian System” was published, that these formations could from that period be recognised and identified in all other parts of Europe and in North America, even in countries where most of the fossils differed specifically from those of the classical region in Britain, where they were first studied.
TABLE 27.1. SHOWING THE SUCCESSION OF THE STRATA IN ENGLAND AND WALES WHICH BELONG TO THE CAMBRIAN GROUP OR THE FOSSILIFEROUS ROCKS OLDER THAN THE ARENIG OR LOWER LLANDEILO ROCKS:
UPPER CAMBRIAN.
TREMADOC SLATES. (Primordial of Barrande in part.)
LINGULA FLAGS. (Primordial of Barrande.)
LOWER CAMBRIAN.
MENEVIAN BEDS. (Primordial of Barrande.)
LONGMYND GROUP.
a. Harlech Grits.
b. Llanberis slates.
While Sir R.I. Murchison was exploring in 1833, in Shropshire and the borders of Wales, the strata which in 1835 he first called Silurian, Professor Sedgwick was surveying the rocks of North Wales, which both these geologists considered at that period as of older date, and for which in 1836 Sedgwick proposed the name of Cambrian. It was afterwards found that a large portion of the slaty rocks of North Wales, which had been considered as more ancient than the Llandeilo beds and Stiper-Stones before alluded to, were, in reality, not inferior in position to those Lower Silurian beds of Murchison, but merely extensive undulations of the same, bearing fossils identical in species, though these were generally rarer and less perfectly preserved, owing to the changes which the rocks had undergone from metamorphic action. To such rocks the term “Cambrian” was no longer applicable, although it continued to be appropriate to strata inferior to the Stiper-Stones, and which were older than those of the Lower Silurian group as originally defined. It was not till 1846 that fossils were found in Wales in the Lingula flags, the place of which will be seen in Table 27.1. By this time Barrande had already published an account of a rich collection of fossils which he had discovered in Bohemia, portions of which he recognised as of corresponding age with Murchison’s Upper and Lower Silurian, while others were more ancient, to which he gave the name of “Primordial,” for the fossils were sufficiently distinct to entitle the rocks to be referred to a new period. They consisted chiefly of trilobites of genera distinct from those occurring in the overlying Silurian formations. These peculiar genera were afterwards found in rocks holding a corresponding position in Wales, and I shall retain for them the term Cambrian, as recent discoveries in our own country seem to carry the first fauna of Barrande, or his primordial type, even into older strata than any which he found to be fossiliferous in Bohemia.
The term primordial was intended to express M. Barrande’s own belief that the fossils of the rocks so-called afforded evidence of the first appearance of vital phenomena on this planet, and that consequently no fossiliferous strata of older date would or could ever be discovered. The acceptance of such a nomenclature would seem to imply that we despaired of extending our discoveries of new and more ancient fossil groups at some future day when vast portions of the globe, hitherto unexplored, should have been thoroughly surveyed. Already the discovery of the Laurentian Eozoon in Canada, presently to be mentioned, discountenances such views.
UPPER CAMBRIAN.
TREMADOC SLATES.
(FIGURE 568. Theca (Cleidotheca) operculata. Lower Tremadoc beds. Tremadoc.)