b. Section of cone showing the position of the seeds.)
Ettingshausen remarked in 1851 that five of the fossil species from Sheppey, named by Bowerbank (Fossil Fruits and Seeds of London Clay Plates 9 and 10.) were specimens of the same fruit (see Figure 206), in different states of preservation; and Mr. Carruthers, having examined the original specimens now in the British Museum, tells me that all these cones from Sheppey may be reduced to two species, which have an undoubted affinity to the two existing Australian genera above mentioned, although their perfect identity in structure can not be made out.
The contiguity of land may be inferred not only from these vegetable productions, but also from the teeth and bones of crocodiles and turtles, since these creatures, as Dean Conybeare remarked, must have resorted to some shore to lay their eggs. Of turtles there were numerous species referred to extinct genera. These are, for the most part, not equal in size to the largest living tropical turtles. A sea-snake, which must have been thirteen feet long, of the genus Palaeophis before mentioned, has also been described by Professor Owen from Sheppey, of a different species from that of Bracklesham, and called Palaeophis toliapicus. A true crocodile, also, Crocodilus toliapicus, and another saurian more nearly allied to the gavial, accompany the above fossils; also the relics of several birds and quadrupeds. One of these last belongs to the new genus Hyracotherium of Owen, of the hog tribe, allied to Chaeropotamus, another is a Lophiodon; a third a pachyderm called Coryphodon eocaenus by Owen, larger than any existing tapir. All these animals seem to have inhabited the banks of the great river which floated down the Sheppey fruits. They imply the existence of a mammiferous fauna antecedent to the period when nummulites flourished in Europe and Asia, and therefore before the Alps, Pyrenees, and other mountain-chains now forming the backbones of great continents, were raised from the deep; nay, even before a part of the constituent rocky masses now entering into the central ridges of these chains had been deposited in the sea.
SHELLS OF THE LONDON CLAY.
(FIGURE 207. Voluta nodosa, Sowerby. Highgate.)
(FIGURE 208. Phorus extensus, Sowerby. Highgate.)
(FIGURE 209. Rostellaria (Hippocrenes) ampla, Brander. 1/3 of natural size; also found in the Barton clay.)
(FIGURE 210. Nautilus centralis, Sowerby. Highgate.)
(FIGURE 211. Aturia ziczac, Bronn. Syn. Nautilus ziczac, Sowerby. London clay. Sheppey.)
(FIGURE 212. Belosepia sepioidea, De Blainv. London clay. Sheppey.)
(FIGURE 213. Leda amygdaloides, Sowerby. Highgate.)
(FIGURE 214. Cyptodon (Axinus) angulatum, Sowerby. London clay. Hornsey.)
(FIGURE 215. Astropecten crispatus, E. Forbes. Sheppey.)
The marine shells of the London Clay confirm the inference derivable from the plants and reptiles in favour of a high temperature. Thus many species of Conus and Voluta occur, a large Cypraea, C. oviformis, a very large Rostellaria (Figure 209), a species of Cancellaria, six species of Nautilus (Figure 211), besides other Cephalopoda of extinct genera, one of the most remarkable of which is the Belosepia (Figure 212). Among many characteristic bivalve shells are Leda amygdaloides (Figure 213) and Cryptodon angulatum (Figure 214), and among the Radiata a star-fish, Astropecten (Figure 215.)
These fossils are accompanied by a sword-fish (Tetrapterus priscus, Agassiz), about eight feet long, and a saw-fish (Pristis bisulcatus, Agassiz), about ten feet in length; genera now foreign to the British seas. On the whole, about eighty species of fish have been described by M. Agassiz from these beds of Sheppey, and they indicate, in his opinion, a warm climate.
In the lower part of the London clay at Kyson, a few miles east of Woodbridge, the remains of mammalia have been detected. Some of these have been referred by Professor Owen to an opossum, and others to the genus Hyracotherium. The teeth of this last-mentioned pachyderm were at first, in 1840, supposed to belong to a monkey, an opinion afterwards abandoned by Owen when more ample materials for comparison were obtained.
WOOLWICH AND READING SERIES (C.2, TABLE 16.1.)
This formation was formerly called the Plastic Clay, as it agrees with a similar clay used in pottery which occupies the same position in the French series, and it has been used for the like purposes in England. (Prestwich Quarterly Geological Journal volume 10.)
No formations can be more dissimilar, on the whole, in mineral character than the Eocene deposits of England and Paris; those of our own island being almost exclusively of mechanical origin– accumulations of mud, sand, and pebbles; while in the neighbourhood of Paris we find a great succession of strata composed of limestones, some of them siliceous, and of crystalline gypsum and siliceous sandstone, and sometimes of pure flint used for millstones. Hence it is often impossible, as before stated, to institute an exact comparison between the various members of the English and French series, and to settle their respective ages. But in regard to the division which we have now under consideration, whether we study it in the basins of London, Hampshire, or Paris, we recognise as a general rule the same mineral character, the beds consisting over a large area of mottled clays and sand, with lignite, and with some strata of well-rolled flint pebbles, derived from the chalk, varying in size, but occasionally several inches in diameter. These strata may be seen in the Isle of Wight in contact with the chalk, or in the London basin, at Reading, Blackheath, and Woolwich. In some of the lowest of them, banks of oysters are observed, consisting of Ostrea bellovacina, so common in France in the same relative position. In these beds at Bromley, Dr. Buckland found a large pebble to which five full-grown oysters were affixed, in such a manner as to show that they had commenced their first growth upon it, and remained attached to it through life.
(FIGURE 216. Cyrena cuneiformis, Sowerby. Natural size. Woolwich clays.)
(FIGURE 217. Melania (Melanatria) inquinata, Des. Syn. Cerithium melanoides, Sowerby. Woolwich clays.)
In several places, as at Woolwich on the Thames, at Newhaven in Sussex, and elsewhere, a mixture of marine and fresh-water testacea distinguishes this member of the series. Among the latter, Cyrena cuneiformis (see Figure 216) and Melania inquinata (see Figure 217) are very common, as in beds of corresponding age in France. They clearly indicate points where rivers entered the Eocene sea. Usually there is a mixture of brackish, fresh-water, and marine shells, and sometimes, as at Woolwich, proofs of the river and the sea having successively prevailed on the same spot. At New Charlton, in the suburbs of Woolwich, Mr. de la Condamine discovered in 1849, and pointed out to me, a layer of sand associated with well-rounded flint pebbles in which numerous individuals of the Cyrena tellinella were seen standing endwise with both their valves united, the siphonal extremity of each shell being uppermost, as would happen if the mollusks had died in their natural position. I have described a bank of sandy mud, in the delta of the Alabama River at Mobile, on the borders of the Gulf of Mexico, where in 1846 I dug out at low tide specimens of living species of Cyrena and of a Gnathodon, which were similarly placed with their shells erect, or in a posture which enables the animal to protrude its siphon upward, and draw in or reject water at pleasure. (Second Visit to the United States volume 2 page 104.) The water at Mobile is usually fresh, but sometimes brackish. At Woolwich a body of river-water must have flowed permanently into the sea where the Cyrenae lived, and they may have been killed suddenly by an influx of pure salt- water, which invaded the spot when the river was low, or when a subsidence of land took place. Traced in one direction, or eastward towards Herne Bay, the Woolwich beds assume more and more of a marine character; while in an opposite, or south-western direction, they become, as near Chelsea and other places, more fresh-water, and contain Unio, Paludina, and layers of lignite, so that the land drained by the ancient river seems clearly to have been to the south-west of the present site of the metropolis.
FLUVIATILE BEDS UNDERLYING DEEP-SEA STRATA.
Before the minds of geologists had become familiar with the theory of the gradual sinking of land, and its conversion into sea at different periods, and the consequent change from shallow to deep water, the fluviatile and littoral character of this inferior group appeared strange and anomalous. After passing through hundreds of feet of London clay, proved by its fossils to have been deposited in deep salt-water, we arrive at beds of fluviatile origin, and associated with them masses of shingle, attaining at Blackheath, near London, a thickness of 50 feet. These shingle banks are probably of marine origin, but they indicate the proximity of land, and the existence of a shore where the flints of the chalk were rolled into sand and pebbles, and spread over a wide space. We have, therefore, first, as before stated, evidence of oscillations of level during the accumulation of the Woolwich series, then of a great submergence, which allowed a marine deposit 500 thick to be laid over the antecedent beds of fresh and brackish water origin.
THANET SANDS (C.3 TABLE 16.1).
The Woolwich or plastic clay above described may often be seen in the Hampshire basin in actual contact with the chalk, constituting in such places the lowest member of the British Eocene series. But at other points another formation of marine origin, characterised by a somewhat different assemblage of organic remains, has been shown by Mr. Prestwich to intervene between the chalk and the Woolwich series. For these beds he has proposed the name of “Thanet Sands,” because they are well seen in the Isle of Thanet, in the northern part of Kent, and on the sea-coast between Herne Bay and the Reculvers, where they consist of sands with a few concretionary masses of sandstone, and contain, among other fossils, Pholadomya cuneata, Cyprina morrisii, Corbula longirostris, Scalaria Bowerbankii, etc. The greatest thickness of these beds is 90 feet.
UPPER EOCENE FORMATIONS OF FRANCE.
The tertiary formations in the neighbourhood of Paris consist of a series of marine and fresh-water strata, alternating with each other, and filling up a depression in the chalk. The area which they occupy has been called the Paris Basin, and is about 180 miles in its greatest length from north to south, and about 90 miles in breadth from east to west. MM. Cuvier and Brongniart attempted, in 1810, to distinguish five different groups, comprising three fresh-water and two marine, which were supposed to imply that the waters of the ocean, and of rivers and lakes, had been by turns admitted into and excluded from the same area. Investigations since made in the Hampshire and London basins have rather tended to confirm these views, at least so far as to show that since the commencement of the Eocene period there have been great movements of the bed of the sea, and of the adjoining lands, and that the superposition of deep-sea to shallow-water deposits (the London Clay, for example, to the Woolwich beds) can only be explained by referring to such movements. It appears, notwithstanding, from the researches of M. Constant Prevost, that some of the minor alternations and intermixtures of fresh-water and marine deposits, in the Paris basin, may be accounted for without such changes of level, by imagining both to have been simultaneously in progress, in the same bay of the same sea, or a gulf into which many rivers entered.
GYPSEOUS SERIES OF MONTMARTRE (A.1, TABLE 16.1).
To enlarge on the numerous subdivisions of the Parisian strata would lead me beyond my present limits; I shall therefore give some examples only of the most important formations. Beneath the Gres de Fontainebleau, belonging to the Lower Miocene period, as before stated, we find, in the neighbourhood of Paris, a series of white and green marls, with subordinate beds of gypsum. These are most largely developed in the central parts of the Paris basin, and, among other places, in the hill of Montmartre, where its fossils were first studied by Cuvier.
The gypsum quarried there for the manufacture of plaster of Paris occurs as a granular crystalline rock, and, together with the associated marls, contains land and fluviatile shells, together with the bones and skeletons of birds and quadrupeds. Several land-plants are also met with, among which are fine specimens of the fan-palm or palmetto tribe (Flabellaria). The remains also of fresh-water fish, and of crocodiles and other reptiles, occur in the gypsum. The skeletons of mammalia are usually isolated, often entire, the most delicate extremities being preserved; as if the carcasses, clothed with their flesh and skin, had been floated down soon after death, and while they were still swollen by the gases generated by their first decomposition. The few accompanying shells are of those light kinds which frequently float on the surface of rivers, together with wood.
In this formation the relics of about fifty species of quadrupeds, including the genera Palaeotherium (see Figure 174), Anoplotherium (see Figure 218), and others, have been found, all extinct, and nearly four-fifths of them belonging to the Perissodactyle or odd-toed division of the order Pachydermata, which now contains only four living genera, namely, rhinoceros, tapir, horse, and hyrax. With them a few carnivorous animals are associated, among which are the Hyaenodon dasyuroides, a species of dog, Canis Parisiensis, and a weasel, Cynodon Parisiensis. Of the Rodentia are found a squirrel; of the Cheiroptera, a bat; while the Marsupalia (an order now confined to America, Australia, and some contiguous islands) are represented by an opossum.
Of birds, about ten species have been ascertained, the skeletons of some of which are entire. None of them are referable to existing species. (Cuvier, Oss. Foss. tome 3 page 255.) The same remark, according to MM. Cuvier and Agassiz, applies both to the reptiles and fish. Among the last are crocodiles and tortoises of the genera Emys and Trionyx.
(FIGURE 218. Xiphodon gracile, or Anoplotherium gracile, Cuvier. Restored outline.)
The tribe of land quadrupeds most abundant in this formation is such as now inhabits alluvial plains and marshes, and the banks of rivers and lakes, a class most exposed to suffer by river inundations. Among these were several species of Palaeotherium, a genus before alluded to. These were associated with the Anoplotherium, a tribe intermediate between pachyderms and ruminants. One of the three divisions of this family was called by Cuvier Xiphodon. Their forms were slender and elegant, and one, named Xiphodon gracile (Figure 218), was about the size of the chamois; and Cuvier inferred from the skeleton that it was as light, graceful, and agile as the gazelle.
FOSSIL FOOTPRINTS.
There are three superimposed masses of gypsum in the neighbourhood of Paris, separated by intervening deposits of laminated marl. In the uppermost of the three, in the valley of Montmorency, M. Desnoyers discovered in 1859 many footprints of animals occurring at no less than six different levels. (Sur des Empreintes de Pas d’Animaux par M. J. Desnoyers. Compte rendu de l’Institut 1859.) The gypsum to which they belong varies from thirty to fifty feet in thickness, and is that which has yielded to the naturalist the largest number of bones and skeletons of mammalia, birds, and reptiles. I visited the quarries, soon after the discovery was made known, with M. Desnoyers, who also showed me large slabs in the Museum at Paris, where, on the upper planes of stratification, the indented foot-marks were seen, while corresponding casts in relief appeared on the lower surfaces of the strata of gypsum which were immediately superimposed. A thin film of marl, which before it was dried and condensed by pressure must have represented a much thicker layer of soft mud, intervened between the beds of solid gypsum. On this mud the animals had trodden, and made impressions which had penetrated to the gypseous mass below, then evidently unconsolidated. Tracks of the Anoplotherium with its bisulcate hoof, and the trilobed footprints of Palaeotherium, were seen of different sizes, corresponding to those of several species of these genera which Cuvier had reconstructed, while in the same beds were foot-marks of carnivorous mammalia. The tracks also of fluviatile, lacustrine, and terrestrial tortoises (Emys, Trionyx, etc.) were discovered, also those of crocodiles, iguanas, geckos, and great batrachians, and the footprints of a huge bird, apparently a wader, of the size of the gastornis, to be mentioned in the sequel. There were likewise the impressions of the feet of other creatures, some of them clearly distinguishable from any of the fifty extinct types of mammalia of which the bones have been found in the Paris gypsum. The whole assemblage, says Desnoyers, indicate the shores of a lake, or several small lakes communicating with each other, on the borders of which many species of pachyderms wandered, and beasts of prey which occasionally devoured them. The tooth-marks of these last had been detected by palaeontologists long before on the bones and skulls of Paleotheres entombed in the gypsum.
IMPERFECTION OF THE RECORD.
These foot-marks have revealed to us new and unexpected proofs that the air- breathing fauna of the Upper Eocene period in Europe far surpassed in the number and variety of its species the largest estimate which had previously been formed of it. We may now feel sure that the mammalia, reptiles, and birds which have left portions of their skeletons as memorials of their existence in the solid gypsum constituted but a part of the then living creation. Similar inferences may be drawn from the study of the whole succession of geological records. In each district the monuments of periods embracing thousands, and probably in some instances hundreds of thousands of years, are totally wanting. Even in the volumes which are extant the greater number of the pages are missing in any given region, and where they are found they contain but few and casual entries of the physical events or living beings of the times to which they relate. It may also be remarked that the subordinate formations met with in two neighbouring countries, such as France and England (the minor Tertiary groups above enumerated), commonly classed as equivalents and referred to corresponding periods, may nevertheless have been by no means strictly coincident in date. Though called contemporaneous, it is probable that they were often separated by intervals of many thousands of years. We may compare them to double stars, which appear single to the naked eye because seen from a vast distance in space, and which really belong to one and the same stellar system, though occupying places in space extremely remote if estimated by our ordinary standard of terrestrial measurements.
CALCAIRE SILICIEUX, OR TRAVERTIN INFERIEUR (A.2 AND 3 TABLE 16.1).
This compact siliceous limestone extends over a wide area. It resembles a precipitate from the waters of mineral springs, and is often traversed by small empty sinuous cavities. It is, for the most part, devoid of organic remains, but in some places contains fresh-water and land species, and never any marine fossils. The calcaire siliceux and the calcaire grossier usually occupy distinct parts of the Paris basin, the one attaining its fullest development in those places where the other is of slight thickness. They are described by some writers as alternating with each other towards the centre of the basin, as at Sergy and Osny.
The gypsum, with its associated marls before described, is in greatest force towards the centre of the basin, where the calcaire grossier and calcaire silicieux are less fully developed.
GRES DE BEAUCHAMP, OR SABLES MOYENS (A.4 TABLE 16.1).
In some parts of the Paris basin, sands and marls, called the Gres de Beauchamp, or Sables moyens, divide the gypseous beds from the calcaire grossier proper. These sands, in which a small nummulite (N. variolaria) is very abundant, contain more than 300 species of marine shells, many of them peculiar, but others common to the next division.
MIDDLE EOCENE FORMATIONS OF FRANCE.
CALCAIRE GROSSIER, UPPER AND MIDDLE (B.1 TABLE 16.1).
The upper division of this group consists in great part of beds of compact, fragile limestone, with some intercalated green marls. The shells in some parts are a mixture of Cerithium, Cyclostoma, and Corbula; in others Limnea, Cerithium, Paludina, etc. In the latter, the bones of reptiles and mammalia, Palaeotherium and Lophiodon, have been found. The middle division, or calcaire grossier proper, consists of a coarse limestone, often passing into sand. It contains the greater number of the fossil shells which characterise the Paris basin. No less than 400 distinct species have been procured from a single spot near Grignon, where they are imbedded in a calcareous sand, chiefly formed of comminuted shells, in which, nevertheless, individuals in a perfect state of preservation, both of marine, terrestrial, and fresh-water species, are mingled together. Some of the marine shells may have lived on the spot; but the Cyclostoma and Limnea, being land and fresh-water shells, must have been brought thither by rivers and currents, and the quantity of triturated shells implies considerable movement in the waters.
Nothing is more striking in this assemblage of fossil testacea than the great proportion of species referable to the genus Cerithium (Figures 160 and 161 Chapter 15). There occur no less than 137 species of this genus in the Paris basin, and almost all of them in the calcaire grossier. Most of the living Cerithia inhabit the sea near the mouths of rivers, where the waters are brackish; so that their abundance in the marine strata now under consideration is in harmony with the hypothesis that the Paris basin formed a gulf into which several rivers flowed.
EOCENE FORAMINIFERA.
(FIGURE 219. Calcarina rarispina, Desh. a. Natural size.
b. Magnified.)
(FIGURE 220. Spirolina stenostoma, Desh. a. Natural size.
b. Magnified.)
(FIGURE 221. Triloculina inflata, Desh. a. Natural size.
b. Magnified.)
In some parts of the calcaire grossier round Paris, certain beds occur of a stone used in building, and called by the French geologists “Miliolite limestone.” It is almost entirely made up of millions of microscopic shells, of the size of minute grains of sand, which all belong to the class Foraminifera. Figures of some of these are given in Figures 219 to 221. As this miliolitic stone never occurs in the Faluns, or Upper Miocene strata of Brittany and Touraine, it often furnishes the geologist with a useful criterion for distinguishing the detached Eocene and Upper Miocene formations scattered over those and other adjoining provinces. The discovery of the remains of Palaeotherium and other mammalia in some of the upper beds of the calcaire grossier shows that these land animals began to exist before the deposition of the overlying gypseous series had commenced.
LOWER CALCAIRE GROSSIER, OR GLAUCONIE GROSSIERE (B.1 TABLE 16.1).
The lower part of the calcaire grossier, which often contains much green earth, is characterised at Auvers, near Pontoise, to the north of Paris, and still more in the environs of Compiegne, by the abundance of nummulites, consisting chiefly of N. laevigata, N. scabra, and N. Lamarcki, which constitute a large proportion of some of the stony strata, though these same foraminifera are wanting in beds of similar age in the immediate environs of Paris.
SOISSONNAIS SANDS, OR LITS COQUILLIERS (B.2 TABLE 16.1).
(FIGURE 222. Nerita conoidea, Lam. Syn. N. Schmidelliana, Chemnitz.)
Below the preceding formation, shelly sands are seen, of considerable thickness, especially at Cuisse-Lamotte, near Compiegne, and other localities in the Soissonnais, about fifty miles N.E. of Paris, from which about 300 species of shells have been obtained, many of them common to the calcaire grossier and the Bracklesham beds of England, and many peculiar. The Nummulites planulata is very abundant, and the most characteristic shell is the Nerita conoidea, Lam., a fossil which has a very wide geographical range; for, as M. d’Archiac remarks, it accompanies the nummulitic formation from Europe to India, having been found in Cutch, near the mouths of the Indus, associated with Nummulites scabra. No less than 33 shells of this group are said to be identical with shells of the London clay proper, yet, after visiting Cuisse-Lamotte and other localities of the “Sables inferieurs” of Archiac, I agree with Mr. Prestwich, that the latter are probably newer than the London clay, and perhaps older than the Bracklesham beds of England. The London clay seems to be unrepresented in the Paris basin, unless partially so, by these sands. (d’Archiac Bulletin tome 10 and Prestwich Quarterly Geological Journal 1847 page 377.)
LOWER EOCENE FORMATIONS OF FRANCE.
ARGILE PLASTIQUE (C.2 TABLE 16.1).
At the base of the tertiary system in France are extensive deposits of sands, with occasional beds of clay used for pottery, and called “argile plastique.” Fossil oysters (Ostrea bellovacina) abound in some places, and in others there is a mixture of fluviatile shells, such as Cyrena cuneiformis (Figure 216), Melania inquinata (Figure 217), and others, frequently met with in beds occupying the same position in the London Basin. Layers of lignite also accompany the inferior clays and sands.
Immediately upon the chalk at the bottom of all the tertiary strata in France there generally is a conglomerate or breccia of rolled and angular chalk-flints, cemented by siliceous sand. These beds appear to be of littoral origin, and imply the previous emergence of the chalk, and its waste by denudation. In the year 1855, the tibia and femur of a large bird equalling at least the ostrich in size were found at Meudon, near Paris, at the base of the Plastic clay. This bird, to which the name of Gastornis Parisiensis has been assigned, appears, from the Memoirs of MM. Hebert, Lartet, and Owen, to belong to an extinct genus. Professor Owen refers it to the class of wading land birds rather than to an aquatic species. (Quarterly Geological Journal volume 12 page 204 1856.)
That a formation so much explored for economical purposes as the Argile plastique around Paris, and the clays and sands of corresponding age near London, should never have afforded any vestige of a feathered biped previously to the year 1855, shows what diligent search and what skill in osteological interpretation are required before the existence of birds of remote ages can be established.
SABLES DE BRACHEUX (C.3 TABLE 16.1).
The marine sands called the Sables de Bracheux (a place near Beauvais), are considered by M. Hebert to be older than the Lignites and Plastic clay, and to coincide in age with the Thanet Sands of England. At La Fere, in the Department of Aisne, in a deposit of this age, a fossil skull has been found of a quadruped called by Blainville Arctocyon primaevus, and supposed by him to be related both to the bear and to the Kinkajou (Cercoleptes). This creature appears to be the oldest known tertiary mammifer.
NUMMULITIC FORMATIONS OF EUROPE, ASIA, ETC.
Of all the rocks of the Eocene period, no formations are of such great geographical importance as the Upper and Middle Eocene, as above defined, assuming that the older tertiary formation, commonly called nummulitic, is correctly ascribed to this group. It appears that of more than fifty species of these foraminifera described by D’Archiac, one or two species only are found in other tertiary formations whether of older or newer date. Nummulites intermedia, a Middle Eocene form, ascends into the Lower Miocene, but it seems doubtful whether any species descends to the level of the London clay, still less to the Argile plastique or Woolwich beds. Separate groups of strata are often characterised by distinct species of nummulite; thus the beds between the lower Miocene and the lower Eocene may be divided into three sections, distinguished by three different species of nummulites, N. variolaria in the upper, N. laevigata in the middle, and N. planulata in the lower beds. The nummulitic limestone of the Swiss Alps rises to more than 10,000 feet above the level of the sea, and attains here and in other mountain chains a thickness of several thousand feet. It may be said to play a far more conspicuous part than any other tertiary group in the solid framework of the earth’s crust, whether in Europe, Asia, or Africa. It occurs in Algeria and Morocco, and has been traced from Egypt, where it was largely quarried of old for the building of the Pyramids, into Asia Minor, and across Persia by Bagdad to the mouths of the Indus. It has been observed not only in Cutch, but in the mountain ranges which separate Scinde from Persia, and which form the passes leading to Caboul; and it has been followed still farther eastward into India, as far as eastern Bengal and the frontiers of China.
(FIGURE 223. Nummulites Puschi, D’Archiac. Peyrehorade, Pyrenees. a. External surface of one of the nummulites, of which longitudinal sections are seen in the limestone.
b. Transverse section of same.)
Dr. T. Thompson found nummulites at an elevation of no less than 16,500 feet above the level of the sea, in Western Thibet. One of the species, which I myself found very abundant on the flanks of the Pyrenees, in a compact crystalline marble (Figure 223) is called by M. D’Archiac Nummulites Puschi. The same is also very common in rocks of the same age in the Carpathians. In many distant countries, in Cutch, for example, some of the same shells, such as Nerita conoidea (Figure 222), accompany the nummulites, as in France. The opinion of many observers, that the Nummulitic formation belongs partly to the cretaceous era, seems chiefly to have arisen from confounding an allied genus, Orbitoides, with the true Nummulite.
When we have once arrived at the conviction that the nummulitic formation occupies a middle and upper place in the Eocene series, we are struck with the comparatively modern date to which some of the greatest revolutions in the physical geography of Europe, Asia, and Northern Africa must be referred. All the mountain-chains, such as the Alps, Pyrenees, Carpathians, and Himalayas, into the composition of whose central and loftiest parts the nummulitic strata enter bodily, could have had no existence till after the Middle Eocene period. During that period the sea prevailed where these chains now rise, for nummulites and their accompanying testacea were unquestionably inhabitants of salt water. Before these events, comprising the conversion of a wide area from a sea to a continent, England had been peopled, as I before pointed out, by various quadrupeds, by herbivorous pachyderms, by insectivorous bats, and by opossums.
Almost all the volcanoes which preserve any remains of their original form, or from the craters of which lava streams can be traced, are more modern than the Eocene fauna now under consideration; and besides these superficial monuments of the action of heat, Plutonic influences have worked vast changes in the texture of rocks within the same period. Some members of the nummulitic and overlying tertiary strata called flysch have actually been converted in the central Alps into crystalline rocks, and transformed into marble, quartz-rock, micha-schist, and gneiss. (Murchison Quarterly Journal of Geological Society volume 5 and Lyell volume 6 1850 Anniversary Address.)
EOCENE STRATA IN THE UNITED STATES.
In North America the Eocene formations occupy a large area bordering the Atlantic, which increases in breadth and importance as it is traced southward from Delaware and Maryland to Georgia and Alabama. They also occur in Louisiana and other States both east and west of the valley of the Mississippi. At Claiborne, in Alabama, no less than 400 species of marine shells, with many echinoderms and teeth of fish, characterise one member of this system. Among the shells, the Cardita planicosta, before mentioned (Figure 191), is in abundance; and this fossil and some others identical with European species, or very nearly allied to them, make it highly probable that the Claiborne beds agree in age with the central or Bracklesham group of England, and with the calcaire grossiere of Paris. (See paper by the Author Quarterly Journal of Geological Society volume 4 page 12 and Second Visit to the United States volume 2 page 59.)
Higher in the series is a remarkable calcareous rock, formerly called “the nummulite limestone,” from the great number of discoid bodies resembling nummulites which it contains, fossils now referred by A. d’Orbigny to the genus Orbitoides, which has been demonstrated by Dr. Carpenter to belong to the foraminifera. (Quarterly Journal of Geological Society volume 6 page 32.) That naturalist, moreover, is of opinion that the Orbitoides alluded to (O. Mantelli) is of the same species as one found in Cutch, in the Middle Eocene or nummulitic formation of India.
Above the orbitoidal limestone is a white limestone, sometimes soft and argillaceous, but in parts very compact and calcareous. It contains several peculiar corals, and a large Nautilus allied to N. ziczac; also in its upper bed a gigantic cetacean, called Zeuglodon by Owen. (See Memoir by R.W. Gibbes Journal of Academy of Natural Science Philadelphia volume 1 1847.)
The colossal bones of this cetacean are so plentiful in the interior of Clarke County, Alabama, as to be characteristic of the formation. The vertebral column of one skeleton found by Dr. Buckley at a spot visited by me, extended to the length of nearly seventy feet, and not far off part of another backbone nearly fifty feet long was dug up. I obtained evidence, during a short excursion, of so many localities of this fossil animal within a distance of ten miles, as to lead me to conclude that they must have belonged to at least forty distinct individuals.
(FIGURE 224. Zeuglodon cetoides, Owen. Basilosaurus, Harlan. Molar tooth, natural size.)
(FIGURE 225. Zeuglodon cetoides, Owen. Basilosaurus, Harlan. Vertebra, reduced.)
Professor Owen first pointed out that this huge animal was not reptilian, since each tooth was furnished with double roots (Figure 224), implanted in corresponding double sockets; and his opinion of the cetacean nature of the fossil was afterwards confirmed by Dr. Wyman and Dr. R.W. Gibbes. That it was an extinct mammal of the whale tribe has since been placed beyond all doubt by discovery of the entire skull of another fossil species of the same family, having the double occipital condyles only met with in mammals, and the convoluted tympanic bones which are characteristic of cetaceans.
CHAPTER XVII.
UPPER CRETACEOUS GROUP.
Lapse of Time between Cretaceous and Eocene Periods. Table of successive Cretaceous Formations. Maestricht Beds.
Pisolitic Limestone of France.
Chalk of Faxoe.
Geographical Extent and Origin of the White Chalk. Chalky Matter now forming in the Bed of the Atlantic. Marked Difference between the Cretaceous and existing Fauna. Chalk-flints.
Pot-stones of Horstead.
Vitreous Sponges in the Chalk.
Isolated Blocks of Foreign Rocks in the White Chalk supposed to be ice-borne. Distinctness of Mineral Character in contemporaneous Rocks of the Cretaceous Epoch.
Fossils of the White Chalk.
Lower White Chalk without Flints.
Chalk Marl and its Fossils.
Chloritic Series or Upper Greensand. Coprolite Bed near Cambridge.
Fossils of the Chloritic Series.
Gault.
Connection between Upper and Lower Cretaceous Strata. Blackdown Beds.
Flora of the Upper Cretaceous Period. Hippurite Limestone.
Cretaceous Rocks in the United States.
We have treated in the preceding chapters of the Tertiary or Cainozoic strata, and have next to speak of the Secondary or Mesozoic formations. The uppermost of these last is commonly called the chalk or the cretaceous formation, from creta, the latin name for that remarkable white earthy limestone, which constitutes an upper member of the group in those parts of Europe where it was first studied. The marked discordance in the fossils of the tertiary, as compared with the cretaceous formations, has long induced many geologists to suspect that an indefinite series of ages elapsed between the respective periods of their origin. Measured, indeed, by such a standard, that is to say, by the amount of change in the Fauna and Flora of the earth effected in the interval, the time between the Cretaceous and Eocene may have been as great as that between the Eocene and Recent periods, to the history of which the last seven chapters have been devoted. Several deposits have been met with here and there, in the course of the last half century, of an age intermediate between the white chalk and the plastic clays and sands of the Paris and London districts, monuments which have the same kind of interest to a geologist which certain medieval records excite when we study the history of nations. For both of them throw light on ages of darkness, preceded and followed by others of which the annals are comparatively well-known to us. But these newly-discovered records do not fill up the wide gap, some of them being closely allied to the Eocene, and others to the Cretaceous type, while none appear as yet to possess so distinct and characteristic a fauna as may entitle them to hold an independent place in the great chronological series.
Among the formations alluded to, the Thanet Sands of Prestwich have been sufficiently described in the last chapter, and classed as Lower Eocene. To the same tertiary series belong the Belgian formations, called by Professor Dumont, Landenian. On the other hand, the Maestricht and Faxoe limestones are very closely connected with the chalk, to which also the Pisolitic limestone of France is referable.
CLASSIFICATION OF THE CRETACEOUS ROCKS.
TABLE 17.1.
UPPER CRETACEOUS OR CHALK PERIOD.
1. Maestricht Beds and Faxoe Limestone. 2. Upper White Chalk, with flints.
3. Lower White Chalk, without flints. 4. Chalk Marl.
5. Chloritic series (or Upper Greensand). 6. Gault.
LOWER CRETACEOUS OR NEOCOMIAN.
1. Marine: Upper Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper part).
2. Marine: Middle Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper part).
3. Marine: Lower Neocomian, see Chapter 18. Fresh-water: Wealden Beds (upper part).
The cretaceous group has generally been divided into an Upper and a Lower series, the Upper called familiarly THE CHALK, and the Lower THE GREENSAND; the one deriving its name from the predominance of white earthy limestone and marl, of which it consists in a great part of France and England, the other or lower series from the plentiful mixture of green or chloritic grains contained in some of the sands and cherts of which it largely consists in the same countries. But these mineral characters often fail, even when we attempt to follow out the same continuous subdivisions throughout a small portion of the north of Europe, and are worse than valueless when we desire to apply them to more distant regions. It is only by aid of the organic remains which characterise the successive marine subdivisions of the formation that we are able to recognise in remote countries, such as the south of Europe or North America, the formations which were there contemporaneously in progress. To the English student of geology it will be sufficient to begin by enumerating those groups which characterise the series in this country and others immediately contiguous, alluding but slightly to those of more distant regions. In Table 17.1 it will be seen that I have used the term Neocomian for that commonly called “Lower Greensand;” as this latter term is peculiarly objectionable, since the green grains are an exception to the rule in many of the members of this group even in districts where it was first studied and named.
MAESTRICHT BEDS.
(FIGURE 226. Belemnitella mucronata, Maestricht, Faxoe, and White Chalk. a. Entire specimen, showing vascular impression on outer surface, and characteristic slit.
b. Section of same, showing place of phragmocone. (For particulars of structure see Chapter 18.))
On the banks of the Meuse, at Maestricht, reposing on ordinary white chalk with flints, we find an upper calcareous formation about 100 feet thick, the fossils of which are, on the whole, very peculiar, and all distinct from tertiary species. Some few are of species common to the inferior white chalk, among which may be mentioned Belemnitella mucronata (Figure 226) and Pecten quadricostatus, a shell regarded by many as a mere variety of Pecten quinquecostatus (see Figure 270). Besides the Belemnite there are other genera, such as Baculites and Hamites, never found in strata newer than the cretaceous, but frequently met with in these Maestricht beds. On the other hand, Voluta, Fasciolaria, and other genera of univalve shells, usually met with only in tertiary strata, occur.
The upper part of the rock, about 20 feet thick, as seen in St. Peter’s Mount, in the suburbs of Maestricht, abounds in corals and Bryozoa, often detachable from the matrix; and these beds are succeeded by a soft yellowish limestone 50 feet thick, extensively quarried from time immemorial for building. The stone below is whiter, and contains occasional nodules of grey chert or chalcedony.
(FIGURE 227. Mosasaurus Camperi. Original more than three feet long.)
(FIGURE 228. Hemipneustes radiatus, Ag. Spatangus radiatus, Lam. Chalk of Maestricht and white chalk.)
M. Bosquet, with whom I examined this formation (August, 1850), pointed out to me a layer of chalk from two to four inches thick, containing green earth and numerous encrinital stems, which forms the line of demarkation between the strata containing the fossils peculiar to Maestricht and the white chalk below. The latter is distinguished by regular layers of black flint in nodules, and by several shells, such as Terebratula carnea (see Figure 246), wholly wanting in beds higher than the green band. Some of the organic remains, however, for which St. Peter’s Mount is celebrated, occur both above and below that parting layer, and, among others, the great marine reptile called Mosasaurus (see Figure 227), a saurian supposed to have been 24 feet in length, of which the entire skull and a great part of the skeleton have been found. Such remains are chiefly met with in the soft freestone, the principal member of the Maestricht beds. Among the fossils common to the Maestricht and white chalk may be instanced the echinoderm, Figure 228.
I saw proofs of the previous denudation of the white chalk exhibited in the lower bed of the Maestricht formation in Belgium, about 30 miles S.W. of Maestricht, at the village of Jendrain, where the base of the newer deposit consisted chiefly of a layer of well-rolled, black chalk-flint pebbles, in the midst of which perfect specimens of Thecidea papillata and Belemnitella mucronata are imbedded. To a geologist accustomed in England to regard rolled pebbles of chalk-flint as a common and distinctive feature of tertiary beds of different ages, it is a new and surprising phenomenon to behold strata made up of such materials, and yet to feel no doubt that they were accumulated in a sea in which the belemnite and other cretaceous mollusca flourished.
PISOLITIC LIMESTONE OF FRANCE.
Geologists were for many years at variance respecting the chronological relations of this rock, which is met with in the neighbourhood of Paris, and at places north, south, east, and west of that metropolis, as between Vertus and Laversines, Meudon and Montereau. By many able palaeontologists the species of fossils, more than fifty in number, were declared to be more Eocene in their appearance than Cretaceous. But M. Hebert found in this formation at Montereau, near Paris, the Pecten quadricostatus, a well-known Cretaceous species, together with some other fossils common to the Maestricht chalk and to the Baculite limestone of the Cotentin, in Normandy. He therefore, as well as M. Alcide d’Orbigny, who had carefully studied the fossils, came to the opinion that it was an upper member of the Cretaceous group. It is usually in the form of a coarse yellowish or whitish limestone, and the total thickness of the series of beds already known is about 100 feet. Its geographical range, according to M. Hebert, is not less than 45 leagues from east to west, and 35 from north to south. Within these limits it occurs in small patches only, resting unconformably on the white chalk.
(FIGURE 229. Portion of Baculites Faujasii. Maestricht and Faxoe beds and white chalk.)
(FIGURE 230. Nautilus Danicus, Schl. Faxoe, Denmark.)
The Nautilus Danicus, Figure 230, and two or three other species found in this rock, are frequent in that of Faxoe, in Denmark, but as yet no Ammonites, Hamites, Scaphites, Turrilites, Baculites, or Hippurites have been met with. The proportion of peculiar species, many of them of tertiary aspect, is confessedly large; and great aqueous erosion suffered by the white chalk, before the pisolitic limestone was formed, affords an additional indication of the two deposits being widely separated in time. The pisolitic formation, therefore, may eventually prove to be somewhat more intermediate in date between the secondary and tertiary epochs than the Maestricht rock.
CHALK OF FAXOE.
In the island of Seeland, in Denmark, the newest member of the chalk series, seen in the sea-cliffs at Stevensklint resting on white chalk with flints, is a yellow limestone, a portion of which, at Faxoe, where it is used as a building stone, is composed of corals, even more conspicuously than is usually observed in recent coral reefs. It has been quarried to the depth of more than 40 feet, but its thickness is unknown. The imbedded shells are chiefly casts, many of them of univalve mollusca, which are usually very rare in the white chalk of Europe. Thus, there are two species of Cypraea, one of Oliva, two of Mitra, four of the genus Cerithium, six of Fusus, two of Trochus, one of Patella, one of Emarginula, etc.; on the whole, more than thirty univalves, spiral or patelliform. At the same time, some of the accompanying bivalve shells, echinoderms, and zoophytes, are specifically identical with fossils of the true Cretaceous series. Among the cephalopoda of Faxoe may be mentioned Baculites Faujasii (Figure 229), and Belemnitella mucronata (Figure 226), shells of the white chalk. The Nautilus Danicus (see Figure 230) is characteristic of this formation; and it also occurs in France in the calcaire pisolitique of Laversin (Department of Oise). The claws and entire skull of a small crab, Brachyurus rugosus (Schlott.), are scattered through the Faxoe stone, reminding us of similar crustaceans inclosed in the rocks of modern coral reefs. Some small portions of this coralline formation consist of white earthy chalk.
COMPOSITION, EXTENT AND ORIGIN OF THE WHITE CHALK.
(FIGURE 231. Diagrammatic section from Hertfordshire, in England, to Sens, in France.
Through London (left), Hythe, Boulogne, Valley of Bray, Paris and Sens (right).)
The highest beds of chalk in England and France consist of a pure, white, calcareous mass, usually too soft for a building-stone, but sometimes passing into a more solid state. It consists, almost purely, of carbonate of lime; the stratification is often obscure, except where rendered distinct by interstratified layers of flint, a few inches thick, occasionally in continuous beds, but oftener in nodules, and recurring at intervals generally from two to four feet distant from each other. This upper chalk is usually succeeded, in the descending order, by a great mass of white chalk without flints, below which comes the chalk marl, in which there is a slight admixture of argillaceous matter. The united thickness of the three divisions in the south of England equals, in some places, 1000 feet. The section in Figure 231 will show the manner in which the white chalk extends from England into France, covered by the tertiary strata described in former chapters, and reposing on lower cretaceous beds.
The area over which the white chalk preserves a nearly homogeneous aspect is so vast, that the earlier geologists despaired of discovering any analogous deposits of recent date. Pure chalk, of nearly uniform aspect and composition, is met with in a north-west and south-east direction, from the north of Ireland to the Crimea, a distance of about 1140 geographical miles, and in an opposite direction it extends from the south of Sweden to the south of Bordeaux, a distance of about 840 geographical miles. In Southern Russia, according to Sir R. Murchison, it is sometimes 600 feet thick, and retains the same mineral character as in France and England, with the same fossils, including Inoceramus Cuvieri, Belemnitella mucronata, and Ostrea vesicularis (Figure 251).
(Figures 232 to 236.– Organic bodies forming the ooze of the bed of the Atlantic at great depths.
(FIGURE 232. Globigerina bulloides. Calcareous Rhizopod.)
(FIGURE 233. Actinocyclus. Siliceous Diatomaceae. )
(FIGURE 234. Pinnularia. Siliceous Diatomaceae.)
(FIGURE 235. Eunotia bidens. Siliceous Diatomaceae.)
(FIGURE 236. Spicula of sponge. Siliceous sponge.))
Great light has recently been thrown upon the origin of the unconsolidated white chalk by the deep soundings made in the North Atlantic, previous to laying down, in 1858, the electric telegraph between Ireland and Newfoundland. At depths sometimes exceeding two miles, the mud forming the floor of the ocean was found, by Professor Huxley, to be almost entirely composed (more than nineteen- twentieths of the whole) of minute Rhizopods, or foraminiferous shells of the genus Globigerina, especially the species Globigerina bulloides (see Figure 232.) the organic bodies next in quantity were the siliceous shells called Polycystineae, and next to them the siliceous skeletons of plants called Diatomaceae (Figures 233, 234, 235), and occasionally some siliceous spiculae of sponges (Figure 236) were intermixed. These were connected by a mass of living gelatinous matter to which he has given the name of Bathybius, and which contains abundance of very minute bodies termed Coccoliths and Coccospheres, which have also been detected fossil in chalk.
Sir Leopold MacClintock and Dr. Wallich have ascertained that 95 per cent of the mud of a large part of the North Atlantic consists of Globigerina shells. But Captain Bullock, R.N., lately brought up from the enormous depth of 16,860 feet a white, viscid, chalky mud, wholly devoid of Globigerinae. This mud was perfectly homogeneous in composition, and contained no organic remains visible to the naked eye. Mr. Etheridge, however, has ascertained by microscopical examination that it is made up of Coccoliths, Discoliths, and other minute fossils like those of the Chalk classed by Huxley as Bathybius, when this term is used in its widest sense. This mud, more than three miles deep, was dredged up in latitude 20 degrees 19′ N., longitude 4 degrees 36′ E., or about midway between Madeira and the Cape of Good Hope.
The recent deep-sea dredgings in the Atlantic conducted by Dr. Wyville Thomson, Dr. Carpenter, Mr. Gwyn Jeffreys, and others, have shown that on the same white mud there sometimes flourish Mollusca, Crustacea, and Echinoderms, besides abundance of siliceous sponges, forming, on the whole, a marine fauna bearing a striking resemblance in its general character to that of the ancient chalk.
POPULAR ERROR AS TO THE GEOLOGICAL CONTINUITY OF THE CRETACEOUS PERIOD.
We must be careful, however, not to overrate the points of resemblance which the deep-sea investigations have placed in a strong light. They have been supposed by some naturalists to warrant a conclusion expressed in these words: “We are still living in the Cretaceous epoch;” a doctrine which has led to much popular delusion as to the bearing of the new facts on geological reasoning and classification. The reader should be reminded that in geology we have been in the habit of founding our great chronological divisions, not on foraminifera and sponges, nor even on echinoderms and corals, but on the remains of the most highly organised beings available to us, such as the mollusca; these being met with, as explained in Chapter 9, in stratified rocks of almost every age. In dealing with the mollusca, it is those of the highest or most specialised organisation, which afford us the best characters in proportion as their vertical range is the most limited. Thus the Cephalopoda are the most valuable, as having a more restricted range in time than the Gasteropoda; and these, again, are more characteristic of the particular stratigraphical subdivisions than are the Lamellibranchiate Bivalves, while these last, again, are more serviceable in classification than the Brachiopoda, a still lower class of shell-fish, which are the most enduring of all.
When told that the new dredgings prove that “we are still living in the Chalk Period,” we naturally ask whether some cuttle-fish has been found with a Belemnite forming part of its internal framework; or have Ammonites, Baculites, Hamites, Turrilites, with four or five other Cephalopodous genera characteristic of the chalk and unknown as tertiary, been met with in the abysses of the ocean? Or, in the absence of these long-extinct forms, has a single spiral univalve, or species of Cretaceous Gasteropod, been found living? Or, to descend still lower in the scale, has some characteristic Cretaceous genus of Lamellibranchiate Bivalve, such as the Inoceramus, or Hippurite, foreign to the Tertiary seas, been proved to have survived down to our time? Or, of the numerous genera of lamellibranchiates common to the Cretaceous and Recent seas, has one species been found living? The answer to all these questions is– not one has been found. Even of the humblest shell-fish, the Brachiopods, no new species common to the Cretaceous and recent seas has yet been met with. It has been very generally admitted by conchologists that out of a hundred species of this tribe occurring fossil in the Upper Chalk– one, and one only, Terebratulina striata, is still living, being thought to be identical with Terebratula caput-serpentis. Although this identity is still questioned by some naturalists of authority, it would certainly not surprise us if another lamp-shell of equal antiquity should be met with in the deep sea.
Had it been declared that we are living in the Eocene epoch, the idea would not be so extravagant, for the great reptiles of the Upper Chalk, the Mosasaurus, Pliosaurus, and Pterodactyle, and many others, as well as so many genera of chambered univalves, had already disappeared from the earth, and the marine fauna had made a greater approach to our own by nearly the entire difference which separates it from the fauna of the Cretaceous seas. The Eocene nummulitic limestone of Egypt is a rock mainly composed, like the more ancient white chalk, of globigerine mud; and if the reader will refer to what we have said of the extent to which the nummulitic marine strata, formed originally at the bottom of the sea, now enter into the framework of mountain chains of the principal continents, he will at once perceive that the present Atlantic, Pacific, and Indian Oceans are geographical terms, which must be wholly without meaning when applied to the Eocene, and still more to the Cretaceous Period; so that to talk of the chalk having been uninterruptedly forming in the Atlantic from the Cretaceous Period to our own, is as inadmissible in a geographical as in a geological sense.
CHALK-FLINTS.
The origin of the layers of flint, whether in the form of nodules, or continuous sheets, or in veins or cracks not parallel to the stratification, has always been more difficult to explain than that of the white chalk. But here, again, the late deep-sea soundings have suggested a possible source of such mineral matter. During the cruise of the “Bulldog,” already alluded to, it was ascertained that while the calcareous Globigerinae had almost exclusive possession of certain tracts of the sea-bottom, they were wholly wanting in others, as between Greenland and Labrador. According to Dr. Wallich, they may flourish in those spaces where they derive nutriment from organic and other matter, brought from the south by the warm waters of the Gulf Stream, and they may be absent where the effects of that great current are not felt. Now, in several of the spaces where the calcareous Rhizopods are wanting, certain microscopic plants, called Diatomaceae, above-mentioned (Figures 233-235), the solid parts of which are siliceous, monopolise the ground at a depth of nearly 400 fathoms, or 2400 feet.
The large quantities of silex in solution required for the formation of these plants may probably arise from the disintegration of feldspathic rocks, which are universally distributed. As more than half of their bulk is formed of siliceous earth, they may afford an endless supply of silica to all the great rivers which flow into the ocean. We may imagine that, after a lapse of many years or centuries, changes took place in the direction of the marine currents, favouring at one time a supply in the same area of siliceous, and at another of calcareous matter in excess, giving rise in the one case to a preponderance of Globigerinae, and in the other of Diatomaceae. These last, and certain sponges, may by their decomposition have furnished the silex, which, separating from the chalky mud, collected round organic bodies, or formed nodules, or filled shrinkage cracks.
POT-STONES.
(FIGURE 237. View of a chalk-pit at Horstead, near Norwich, showing the position of the pot-stones. From a drawing by Mrs. Gunn.)
A more difficult enigma is presented by the occurrence of certain huge flints, or pot-stones, as they are called in Norfolk, occurring singly, or arranged in nearly continuous columns at right angles to the ordinary and horizontal layers of small flints. I visited in the year 1825 an extensive range of quarries then open on the river Bure, near Horstead, about six miles from Norwich, which afforded a continuous section, a quarter of a mile in length, of white chalk, exposed to the depth of about twenty-six feet, and covered by a bed of gravel. The pot-stones, many of them pear-shaped, were usually about three feet in height and one foot in their transverse diameter, placed in vertical rows, like pillars, at irregular distances from each other, but usually from twenty to thirty feet apart, though sometimes nearer together, as in Figure 237. These rows did not terminate downward in any instance which I could examine, nor upward, except at the point where they were cut off abruptly by the bed of gravel. On breaking open the pot-stones, I found an internal cylindrical nucleus of pure chalk, much harder than the ordinary surrounding chalk, and not crumbling to pieces like it, when exposed to the winter’s frost. At the distance of half a mile, the vertical piles of pot-stones were much farther apart from each other. Dr. Buckland has described very similar phenomena as characterising the white chalk on the north coast of Antrim, in Ireland. (Geological Transactions 1st Series volume 4 page 413.)
VITREOUS SPONGES OF THE CHALK.
These pear-shaped masses of flint often resemble in shape and size the large sponges called Neptune’s Cups (Spongia patera, Hardw.), which grow in the seas of Sumatra; and if we could suppose a series of such gigantic sponges to be separated from each other, like trees in a forest, and the individuals of each successive generation to grow on the exact spot where the parent sponge died and was enveloped in calcareous mud, so that they should become piled one above the other in a vertical column, their growth keeping pace with the accumulation of the enveloping calcareous mud, a counterpart of the phenomena of the Horstead pot-stones might be obtained.
(FIGURE 238. Ventriculites radiatus, Mantell. Syn. Ocellaria radiata. D’Orbigny. White chalk.)
Professor Wyville Thomson, describing the modern soundings in 1869 off the north coast of Scotland, speaks of the ooze or chalk mud brought from a depth of about 3000 feet, and states that at one haul they obtained forty specimens of vitreous sponges buried in the mud. He suggests that the Ventriculites of the chalk were nearly allied to these sponges, and that when the silica of their spicules was removed, and was dissolved out of the calcareous matrix, it set into flint.
BOULDERS AND GROUPS OF PEBBLES IN CHALK.
The occurrence here and there, in the white chalk of the south of England, of isolated pebbles of quartz and green schist has justly excited much wonder. It was at first supposed that they had been dropped from the roots of some floating tree, by which means stones are carried to some of the small coral islands of the Pacific. But the discovery in 1857 of a group of stones in the white chalk near Croydon, the largest of which was syenite and weighed about forty pounds, accompanied by pebbles and fine sand like that of a beach, has been shown by Mr. Godwin Austen to be inexplicable except by the agency of floating ice. If we consider that icebergs now reach 40 degrees north latitude in the Atlantic, and several degrees nearer the equator in the southern hemisphere, we can the more easily believe that even during the Cretaceous epoch, assuming that the climate was milder, fragments of coast ice may have floated occasionally as far as the south of England.
DISTINCTNESS OF MINERAL CHARACTER IN CONTEMPORANEOUS ROCKS OF THE CRETACEOUS PERIOD.
But we must not imagine that because pebbles are so rare in the white chalk of England and France there are no proofs of sand, shingle, and clay having been accumulated contemporaneously even in European seas. The siliceous sandstone called “upper quader” by the Germans overlies white argillaceous chalk or “planer-kalk,” a deposit resembling in composition and organic remains the chalk marl of the English series. This sandstone contains as many fossil shells common to our white chalk as could be expected in a sea-bottom formed of such different materials. It sometimes attains a thickness of 600 feet, and, by its jointed structure and vertical precipices, plays a conspicuous part in the picturesque scenery of Saxon Switzerland, near Dresden. It demonstrates that in the Cretaceous sea, as in our own, distinct mineral deposits were simultaneously in progress. The quartzose sandstone alluded to, derived from the detritus of the neighbouring granite, is absolutely devoid of carbonate of lime, yet it was formed at the distance only of four hundred miles from a sea-bottom now constituting part of France, where the purely calcareous white chalk was forming. In the North American continent, on the other hand, where the Upper Cretaceous formations are so widely developed, true white chalk, in the ordinary sense of that term, does not exist.
FOSSILS OF THE WHITE CHALK.
(FIGURE 239. Ananchytes ovatus, Leske. White chalk, upper and lower. a. Side view.
b. Base of the shell, on which both the oral and anal apertures are placed; the anal being more round, and at the smaller end.)
(FIGURE 240. Micraster cor-angumum, Leske. White chalk.)
(FIGURE 241. Galerites albogalerus, Lam. White chalk.)
(FIGURE 242. Marsupites Milleri. Mant. White chalk.)
Among the fossils of the white chalk, echinoderms are very numerous; and some of the genera, like Ananchytes (see Figure 239), are exclusively cretaceous. Among the Crinoidea, the Marsupites (Figure 242) is a characteristic genus. Among the mollusca, the cephalopoda are represented by Ammonites, Baculites (Figure 229), and Belemnites (Figure 226). Although there are eight or more species of Ammonites and six of them peculiar to it, this genus is much less fully represented than in each of the other subdivisions of the Upper Cretaceous group.
(FIGURE 243. Terebratulina striata, Wahlenb. Upper white chalk.)
(FIGURE 244. Rhynchonella octoplicata, Sowerby. (Var. of R. plicatilis). Upper white chalk.
(FIGURE 245. Magas pumila, Sowerby. Upper white chalk.)
(FIGURE 246. Terebratula carnea, Sowerby. Upper white chalk.)
(FIGURE 247. Terebratula biplicata, Brocch. Upper cretaceous.)
(FIGURE 248. Crania Parisiensis, Duf. Inferior or attached valve. Upper white chalk.)
(FIGURE 249. Pecten Beaveri, Sowerby. Reduced to one-third diameter. Lower white chalk and chalk marl. Maidstone.)
(FIGURE 250. Lima spinosa, Sowerby. Syn. Spondylus spinosus. Upper white chalk.)
(FIGURE 251. Ostrea vesicularis. Syn. Gryphaea convexa. Upper chalk and upper greensand.)
Among the brachiopoda in the white chalk, the Terebratulae are very abundant (see Figures 243-247). With these are associated some forms of oyster (see Figure 251), and other bivalves (Figures 249, 250).
(FIGURE 252. Inoceramus Lamarckii. Syn. Catillus Lamarckii. White chalk (Dixon’s Geology Sussex Table 28 Figure 29).)
Among the bivalve mollusca, no form marks the Cretaceous era in Europe, America, and India in a more striking manner than the extinct genus Inoceramus (Catillus of Lam.; see Figure 252), the shells of which are distinguished by a fibrous texture, and are often met with in fragments, having probably been extremely friable.
(Figures 253 to 256. Radiolites Mortoni. Mantell. Houghton, Sussex. White chalk. Diameter one-seventh natural size. On the side where the shell is thinnest, there is one external furrow and corresponding internal ridge, a, b, Figures 253, 254; but they are usually less prominent than in these figures. The upper or opercular valve is wanting.
(FIGURE 253. Two individuals deprived of their upper valves, adhering together.)
(FIGURE 254. Same seen from above.)
(FIGURE 255. Transverse section of part of the wall of the shell, magnified to show the structure.)
(FIGURE 256. Vertical section of the same.))
Of the singular family called Rudistes by Lamarck, hereafter to be mentioned as extremely characteristic of the chalk of southern Europe, a single representative only (Figure 253) has been discovered in the white chalk of England.
(FIGURE 257. Eschara disticha. White chalk. a. Natural size.
b. Portion magnified.)
(FIGURE 258. Escharina oceani.
a. Natural size.
b. Part of the same magnified.
White chalk.)
(FIGURE 259. A branching sponge in a flint, from the white chalk. From the collection of Mr. Bowerbank.)
The general absence of univalve mollusca in the white chalk is very marked. Of bryozoa there is an abundance, such as Eschara and Escharina (Figures 257, 258). These and other organic bodies, especially sponges, such as Ventriculites (Figure 238), are dispersed indifferently through the soft chalk and hard flint, and some of the flinty nodules owe their irregular forms to inclosed sponges, such as Figure 259, a, where the hollows in the exterior are caused by the branches of a sponge (Figure 259, b), seen on breaking open the flint.
(FIGURE 260. Palatal tooth of Ptychodus decurrens. Lower white chalk. Maidstone.)
(FIGURE 261. Cestracion Phillippi; recent. Port Jackson. Buckland, Bridgewater Treatise Plate 27 d.))
The remains of fishes of the Upper Cretaceous formations consist chiefly of teeth belonging to the shark family. Some of the genera are common to the Tertiary formations, and some are distinct. To the latter belongs the genus Ptychodus (Figure 260), which is allied to the living Port Jackson shark, Cestracion Phillippi, the anterior teeth of which (see Figure 261, a) are sharp and cutting, while the posterior or palatal teeth (b) are flat (Figure 260). But we meet with no bones of land-animals, nor any terrestrial or fluviatile shells, nor any plants, except sea-weeds, and here and there a piece of drift-wood. All the appearances concur in leading us to conclude that the white chalk was the product of an open sea of considerable depth.
The existence of turtles and oviparous saurians, and of a Pterodactyl or winged lizard, found in the white chalk of Maidstone, implies, no doubt, some neighbouring land; but a few small islets in mid-ocean, like Ascension, formerly so much frequented by migratory droves of turtle, might perhaps have afforded the required retreat where these creatures laid their eggs in the sand, or from which the flying species may have been blown out to sea. Of the vegetation of such islands we have scarcely any indication, but it consisted partly of cycadaceous plants; for a fragment of one of these was found by Captain Ibbetson in the Chalk Marl of the Isle of Wight, and is referred by A. Brongniart to Clathraria Lyellii, Mantell, a species common to the antecedent Wealden period. The fossil plants, however, of beds corresponding in age to the white chalk at Aix-la-Chapelle, presently to be described, like the sandy beds of Saxony, before alluded to, afford such evidence of land as to prove how vague must be any efforts of ours to restore the geography of that period.
The Pterodactyl of the Kentish chalk, above alluded to, was of gigantic dimensions, measuring 16 feet 6 inches from tip to tip of its outstretched wings. Some of its elongated bones were at first mistaken by able anatomists for those of birds; of which class no osseous remains have as yet been derived from the white chalk, although they have been found (as will be seen) in the Chloritic sand.
(FIGURE 262. Coprolites of fish, from the chalk.)
The collector of fossils from the white chalk was formerly puzzled by meeting with certain bodies which they call larch-cones, which were afterwards recognised by Dr. Buckland to be the excrement of fish (see Figure 262). They are composed in great part of phosphate of lime.
LOWER WHITE CHALK.
(FIGURE 263. Baculites anceps, Lam. Lower chalk.)
The Lower White Chalk, which is several hundred feet thick, without flints, has yielded 25 species of Ammonites, of which half are peculiar to it. The genera Baculite, Hamite, Scaphite, Turrilite, Nautilus, Belemnite, and Belemnitella, are also represented.
CHALK MARL.
(FIGURE 264. Ammonites Rhotomagensis. Chalk marl. Back and side view.)
(FIGURE 265. Turrilites costatus, Lam. Lower chalk and chalk marl. a. Section, showing the foliated border of the sutures of the chambers.)
(FIGURE 266. Scaphites aequalis. Chloritic marl and sand, Dorsetshire.)
The lower chalk without flints passes gradually downward, in the south of England, into an argillaceous limestone, “the chalk marl,” already alluded to. It contains 32 species of Ammonites, seven of which are peculiar to it, while eleven pass up into the overlying lower white chalk. A. Rhotomagensis is characteristic of this formation. Among the British cephalopods of other genera may be mentioned Scaphites aequalis (Figure 266) and Turrilites costatus (Figure 265).
CHLORITIC SERIES (OR UPPER GREENSAND).
According to the old nomenclature, this subdivision of the chalk was called Upper Greensand, in order to distinguish it from those members of the Neocomian or Lower Cretaceous series below the Gault to which the name of Greensand had been applied. Besides the reasons before given for abandoning this nomenclature, it is objectionable in this instance as leading the uninitiated to suppose that the divisions thus named Upper and Lower Greensand are of co-ordinate value, instead of which the chloritic sand is quite a subordinate member of the Upper Cretaceous group, and the term Greensand has very commonly been used for the whole of the Lower Cretaceous rocks, which are almost comparable in importance to the entire Upper Cretaceous series. The higher portion of the Chloritic series in some districts has been called chloritic marl, from its consisting of a chalky marl with chloritic grains. In parts of Surrey, where calcareous matter is largely intermixed with sand, it forms a stone called malm-rock or firestone. In the cliffs of the southern coast of the Isle of Wight it contains bands of calcareous limestone with nodules of chert.
COPROLITE BED.
The so-called coprolite bed, found near Farnham, in Surrey, and near Cambridge, contains nodules of phosphate of lime in such abundance as to be largely worked for the manufacture of artificial manure. It belongs to the upper part of the Chloritic series, and is doubtless chiefly of animal origin, and may perhaps be partly coprolitic, derived from the excrement of fish and reptiles. The late Mr. Barrett discovered in it, near Cambridge, in 1858, the remains of a bird, which was rather larger than the common pigeon, and probably of the order Natatores, and which, like most of the Gull tribe, had well-developed wings. Portions of the metacarpus, metatarsus, tibia, and femur have been detected, and the determinations of Mr. Barrett have been confirmed by Professor Owen.
This phosphatic bed in the suburbs of Cambridge must have been formed partly by the denudation of pre-existing rocks, mostly of Cretaceous age. The fossil shells and bones of animals washed out of these denuded strata, now forming a layer only a few feet thick, have yielded a rich harvest to the collector. A large Rudist of the genus Radiolite, no less than two feet in height, may be seen in the Cambridge Museum, obtained from this bed. The number of reptilian remains, all apparently of Cretaceous age, is truly surprising; more than ten species of Pterodactyl, five or six of Ichthyosaurus, one of Pliosaurus, one of Dinosaurus, eight of Chelonians, besides other forms, having been recognised.
The chloritic sand is regarded by many geologists as a littoral deposit of the Chalk Ocean, and therefore contemporaneous with part of the chalk marl, and even, perhaps, with some part of the white chalk. For, as the land went on sinking, and the cretaceous sea widened its area, white mud and chloritic sand were always forming somewhere, but the line of sea-shore was perpetually shifting its position. Hence, though both sand and mud originated simultaneously, the one near the land, the other far from it, the sands in every locality where a shore became submerged might constitute the underlying deposit.
(FIGURE 267. Ostrea columba. Syn. Gryphaea columba. Chloritic sand.)
(FIGURE 268. Ostrea carinata. Chalk marl and chloritic sand. Neocomian.)
(FIGURE 269. Terebrirostra lyra, Sowerby. Chloritic sand.)
(FIGURE 270. Pecten 5-costatus. White chalk and chloritic sand. Neocomian.)
(FIGURE 271. Plagiostoma Hoperi, Sowerby. Syn. Lima Hoperi. White chalk and chloritic sand.)
Among the characteristic mollusca of the chloritic sand may be mentioned Terebrirostra lyra (Figure 269), Plagiostoma Hoperi (Figure 271), Pecten quinque-costatus (Figure 270), and Ostrea columba (Figure 267).
The Cephalopoda are abundant, among which 40 species of Ammonites are now known, 10 being peculiar to this subdivision, and the rest common to the beds immediately above or below.
GAULT.
(FIGURE 272. Ancyloceras spinigerum, d’Orb. Syn. Hamites spiniger, Sowerby. Near Folkestone. Gault.)
The lowest member of the Upper Cretaceous group, usually about 100 feet thick in the S.E. of England, is provincially termed Gault. It consists of a dark blue marl, sometimes intermixed with green sand. Many peculiar forms of cephalopoda, such as the Hamite (Figure 272), and Scaphite, with other fossils, characterise this formation, which, small as is its thickness, can be traced by its organic remains to distant parts of Europe, as, for example, to the Alps.
Twenty-one species of British Ammonites are recorded as found in the Gault, of which only eight are peculiar to it, ten being common to the overlying Chloritic series.
CONNECTION BETWEEN UPPER AND LOWER CRETACEOUS STRATA.– BLACKDOWN BEDS.
The break between the Upper and Lower Cretaceous formations will be appreciated when it is stated that, although the Neocomian contains 31 species of Ammonite, and the Gault, as we have seen, 21, there are only three of those common to both divisions. Nevertheless, we may expect the discovery in England, and still more when we extend our survey to the Continent, of beds of passage intermediate between the Upper and Lower Cretaceous. Even now the Blackdown beds in Devonshire, which rest immediately on Triassic strata, and which evidently belong to some part of the Cretaceous series, have been referred by some geologists to the Upper group, by others to the Lower or Neocomian. They resemble the Folkestone beds of the latter series in mineral character, and 59 out of 156 of their fossil mollusca are common to them; but they have also 16 species common to the Gault, and 20 to the overlying Chloritic series; and what is very important, out of seven Ammonites six are found also in the Gault and Chloritic series, only one being peculiar to the Blackdown beds.
Professor Ramsay has remarked that there is a stratigraphical break; for in Kent, Surrey, and Sussex, at those few points where there are exposures of junctions of the Gault and Neocomian, the surface of the latter has been much eroded or denuded, while to the westward of the great chalk escarpment the unconformability of the two groups is equally striking. At Blackdown this unconformability is still more marked, for though distant only 100 miles from Kent and Surrey, no formation intervenes between these beds and the Trias; all intermediate groups, such as the Lower Neocomian and Oolite, having either not been deposited or destroyed by denudation.
FLORA OF THE UPPER CRETACEOUS PERIOD.
As the Upper Cretaceous rocks of Europe are, for the most part, of purely marine origin, and formed in deep water usually far from the nearest shore, land-plants of this period, as we might naturally have anticipated, are very rarely met with. In the neighbourhood of Aix-la-Chapelle, however, an important exception occurs, for there certain white sands and laminated clays, 400 feet in thickness, contain the remains of terrestrial plants in a beautiful state of preservation. These beds are the equivalents of the white chalk and chalk marl of England, or Senonien of d’Orbigny, although the white siliceous sands of the lower beds, and the green grains in the upper part of the formation, cause it to differ in mineral character from our white chalk.
Beds of fine clay, with fossil plants, and with seams of lignite, and even perfect coal, are intercalated. Floating wood, containing perforating shells, such as Pholas and Gastrochoena, occur. There are likewise a few beds of a yellowish-brown limestone, with marine shells, which enable us to prove that the lowest and highest plant-beds belong to one group. Among these shells are Pecten quadricostatus, and several others which are common to the upper and lower part of the series, and Trigonia limbata, d’Orbigny, a shell of the white chalk. On the whole, the organic remains and the geological position of the strata prove distinctly that in the neighbourhood of Aix-la-Chapelle a gulf of the ancient Cretaceous sea was bounded by land composed of Devonian rocks. These rocks consisted of quartzose and schistose beds, the first of which supplied white sand and the other argillaceous mud to a river which entered the sea at this point, carrying down in its turbid waters much drift-wood and the leaves of plants. Occasionally, when the force of the river abated, marine shells of the genera Trigonia, Turritella, Pecten, etc., established themselves in the same area, and plants allied to Zostera and Fucus grew on the bottom.
The fossil plants of this member of the upper chalk at Aix have been diligently collected and studied by Dr. Debey, and as they afford the only example yet known of a terrestrial flora older than the Eocene, in which the great divisions of the vegetable kingdom are represented in nearly the same proportions as in our own times, they deserve particular attention. Dr. Debey estimates the number of species as amounting to more than two hundred, of which sixty-seven are cryptogamous, chiefly ferns, twenty species of which can be well determined, most of them being in fructification. The scars on the bark of one or two are supposed to indicate tree-ferns. Of thirteen genera three are still existing, namely, Gleichenia, now inhabiting the Cape of Good Hope, and New Holland; Lygodium, now spread extensively through tropical regions, but having some species which live in Japan and North America; and Asplenium, a cosmopolite form. Among the phaenogamous plants, the Conifers are abundant, the most common belonging to a genus called Cycadopteris by Debey, and hardly separable from Sequoia (or Wellingtonia), of which both the cones and branches are preserved. When I visited Aix, I found the silicified wood of this plant very plentifully dispersed through the white sands in the pits near that city. In one silicified trunk 200 rings of annual growth could be counted. Species of Araucaria like those of Australia are also found. Cycads are extremely rare, and of Monocotyledons there are but few. No palms have been recognised with certainty, but the genus Pandanus, or screw pine, has been distinctly made out. The number of the Dicotyledonous Angiosperms is the most striking feature in so ancient a flora.
(In this and subsequent remarks on fossil plants I shall often use Dr. Lindley’s terms, as most familiar in this country; but as those of M. A. Brongniart are much cited, it may be useful to geologists to give a table explaining the corresponding names of groups so much spoken of in palaeontology.
COLUMN 1. BRONGNIART.
COLUMN 2. LINDLEY.
COLUMN 3. EXAMPLES.
CRYPTOGAMIC.
1. Cryptogamous amphigens, or cellular cryptogamic: Thallogens: Lichens, sea- weeds, fungi.
2. Cryptogamous acrogens: Acrogens: Mosses, equisetums, ferns, lycopodiums– Lepidodendra.
PHAENEROGAMIC.
3. Dicotyledonous gymnosperms: Gymnogens: Conifers and Cycads.
4. Dicotyledonous Angiosperms: Exogens: Compositae, leguminosae, umbelliferae, cruciferae, heaths, etc. All native European trees except conifers.
5. Monocotyledons: Endogens. Palms, lilies, aloes, rushes, grasses, etc.)
Among them we find the familiar forms of the Oak, Fig, and Walnut (Quercus, Ficus, and Juglans), of the last both the nuts and leaves; also several genera of the Myrtaceae. But the predominant order is the Proteaceae, of which there are between sixty and seventy supposed species, many of extinct genera, but some referred to the following living forms– Dryandra, Grevillea, Hakea, Banksia, Persoonia– all now belonging to Australia, and Leucospermum, species of which form small bushes at the Cape.
The epidermis of the leaves of many of these Aix plants, especially of the Proteaceae, is so perfectly preserved in an envelope of fine clay, that under the microscope the stomata, or polygonal cellules, can be detected, and their peculiar arrangement is identical with that known to characterise some living Proteaceae (Grevillea, for example). Although this peculiarity of the structure of stomata is also found in plants of widely distant orders, it is, on the whole, but rarely met with, and being thus observed to characterise a foliage previously suspected to be proteaceous, it adds to the probability that the botanical evidence had been correctly interpreted.
An occasional admixture at Aix-la-Chapelle of Fucoids and Zosterites attests, like the shells, the presence of salt-water. Of insects, Dr. Debey has obtained about ten species of the families Curculionidae and Carabidae.
The resemblance of the flora of Aix-la-Chapelle to the tertiary and living floras in the proportional number of dicotyledonous angiosperms as compared to the gymnogens, is a subject of no small theoretical interest, because we can now affirm that these Aix plants flourished before the rich reptilian fauna of the secondary rocks had ceased to exist. The Ichthyosaurus, Pterodactyl, and Mosasaurus were of coeval date with the oak, the walnut, and the fig. Speculations have often been hazarded respecting a connection between the rarity of Exogens in the older rocks and a peculiar state of the atmosphere. A denser air, it was suggested, had in earlier times been alike adverse to the well-being of the higher order of flowering plants, and of the quick-breathing animals, such as mammalia and birds, while it was favourable to a cryptogamic and gymnospermous flora, and to a predominance of reptile life. But we now learn that there is no incompatibility in the co-existence of a vegetation like that of the present globe, and some of the most remarkable forms of the extinct reptiles of the age of gymnosperms.
If the passage seem at present to be somewhat sudden from the flora of the Lower or Neocomian to that of the Upper Cretaceous period, the abruptness of the change will probably disappear when we are better acquainted with the fossil vegetation of the uppermost beds of the Neocomian and that of the lowest strata of the Gault or true Cretaceous series.
HIPPURITE LIMESTONE.– DIFFERENCE BETWEEN THE CHALK OF THE NORTH AND SOUTH OF EUROPE.
(FIGURE 273. Map of part of S.W. France, from the Loire river to the Pyrenees.)
By the aid of the three tests, superposition, mineral character, and fossils, the geologist has been enabled to refer to the same Cretaceous period certain rocks in the north and south of Europe, which differ greatly both in their fossil contents and in their mineral composition and structure.
If we attempt to trace the cretaceous deposits from England and France to the countries bordering the Mediterranean, we perceive, in the first place, that in the neighbourhood of London and Paris they form one great continuous mass, the Straits of Dover being a trifling interruption, a mere valley with chalk cliffs on both sides. We then observe that the main body of the chalk which surrounds Paris stretches from Tours to near Poitiers (see Figure 273, in which the shaded part represents chalk).
Between Poitiers and La Rochelle, the space marked A on the map separates two regions of chalk. This space is occupied by the Oolite and certain other formations older than the Chalk and Neocomian, and has been supposed by M. E. de Beaumont to have formed an island in the Cretaceous sea. South of this space we again meet with rocks which we at once recognise to be cretaceous, partly from the chalky matrix and partly from the fossils being very similar to those of the white chalk of the north: especially certain species of the genera Spatangus, Ananchytes, Cidarites, Nucula, Ostrea, Gryphaea (Exogyra), Pecten, Plagiostoma (Lima), Trigonia, Catillus (Inoceramus), and Terebratula. (d’Archiac, Sur la form. Cretacee du S.-O. de la France Mem. de la Soc. Geol. de France tome 2.) But Ammonites, as M. d’Archiac observes, of which so many species are met with in the chalk of the north of France, are scarcely ever found in the southern region; while the genera Hamite, Turrilite, and Scaphite, and perhaps Belemnite, are entirely wanting.
(FIGURE 274. Radiolites radiosa, d’Orbigny. White chalk of France. b. Upper valve of same.)
(FIGURE 275. Radiolites foliaceus, d’Orbigny. Syn. Sphaerulites agarici-formis, Blainv. White chalk of France.)
(FIGURE 276. Hippurites organisans, Desmoulins. Upper chalk:– chalk marl of Pyrenees? (d’Orbigny’s Palaeontologie francaise plate 533.) a. Young individual; when full grown they occur in groups adhering laterally to each other.
b. Upper side of the upper valve, showing a reticulated structure in those parts, b, where the external coating is worn off. c. Upper end or opening of the lower and cylindrical valve. d. Cast of the interior of the lower conical valve.)
On the other hand, certain forms are common in the south which are rare or wholly unknown in the north of France. Among these may be mentioned many Hippurites, Sphaerulites, and other members of that great family of mollusca called Rudistes by Lamarck, to which nothing analogous has been discovered in the living creation, but which is quite characteristic of rocks of the Cretaceous era in the south of France, Spain, Sicily, Greece, and other countries bordering the Mediterranean. The species called Hippurites organisans (Figure 276) is more abundant than any other in the south of Europe; and the geologist should make himself well acquainted with the cast of the interior, d, which is often the only part preserved in many compact marbles of the Upper Cretaceous period. The flutings on the interior of the Hippurite, which are represented on the cast by smooth, rounded longitudinal ribs, and in some individuals attain a great size and length, are wholly unlike the markings on the exterior of the shell.
CRETACEOUS ROCKS IN THE UNITED STATES.
If we pass to the American continent, we find in the State of New Jersey a series of sandy and argillaceous beds wholly unlike in mineral character to our Upper Cretaceous system; which we can, nevertheless, recognise as referable, palaeontologically, to the same division.
That they were about the same age generally as the European chalk and Neocomian, was the conclusion to which Dr. Morton and Mr. Conrad came after their investigation of the fossils in 1834. The strata consist chiefly of green sand and green marl, with an overlying coralline limestone of a pale yellow colour, and the fossils, on the whole, agree most nearly with those of the Upper European series, from the Maestricht beds to the Gault inclusive. I collected sixty shells from the New Jersey deposits in 1841, five of which were identical with European species– Ostrea larva, O. vesicularis, Gryphaea costata, Pecten quinque-costatus, Belemnitella mucronata. As some of these have the greatest vertical range in Europe, they might be expected more than any others to recur in distant parts of the globe. Even where the species were different, the generic forms, such as the Baculite and certain sections of Ammonites, as also the Inoceramus (see above, Figure 252) and other bivalves, have a decidedly cretaceous aspect. Fifteen out of the sixty shells above alluded to were regarded by Professor Forbes as good geographical representatives of well-known cretaceous fossils of Europe. The correspondence, therefore, is not small, when we reflect that the part of the United States where these strata occur is between 3000 and 4000 miles distant from the chalk of Central and Northern Europe, and that there is a difference of ten degrees in the latitude of the places compared on opposite sides of the Atlantic. Fish of the genera Lamna, Galeus, and Carcharodon are common to New Jersey and the European cretaceous rocks. So also is the genus Mosasaurus among reptiles.
It appears from the labours of Dr. Newberry and others, that the Cretaceous strata of the United States east and west of the Appalachians are characterised by a flora decidedly analogous to that of Aix-la-Chapelle above-mentioned, and therefore having considerable resemblance to the vegetation of the Tertiary and Recent Periods.
CHAPTER XVIII.
LOWER CRETACEOUS OR NEOCOMIAN FORMATION.
Classification of marine and fresh-water Strata. Upper Neocomian.
Folkestone and Hythe Beds.
Atherfield Clay.
Similarity of Conditions causing Reappearance of Species after short Intervals. Upper Speeton Clay.
Middle Neocomian.
Tealby Series.
Middle Speeton Clay.
Lower Neocomian.
Lower Speeton Clay.
Wealden Formation.
Fresh-water Character of the Wealden. Weald Clay.
Hastings Sands.
Punfield Beds of Purbeck, Dorsetshire. Fossil Shells and Fish of the Wealden.
Area of the Wealden.
Flora of the Wealden.
We now come to the Lower Cretaceous Formation which was formerly called Lower Greensand, and for which it will be useful for reasons before explained (Chapter 17) to use the term “Neocomian.”
TABLE 18.1. LOWER CRETACEOUS OR NEOCOMIAN GROUP.
COLUMN 1: MARINE.
COLUMN 2: FRESH-WATER.
1. Upper Neocomian– Greensand of Folkestone, Sandgate, and Hythe, Atherfield clay, upper part of Speeton clay: Part of Wealden beds of Kent, Surrey, Sussex, Hants, and Dorset.
2. Middle Neocomian– Punfield Marine bed, Tealby beds, middle part of Speeton clay: Part of Wealden beds of Kent, Surrey, Sussex, Hants, and Dorset.
3. Lower Neocomian– Lower part of Speeton clay: Part of Wealden beds of Kent, Surrey, Sussex, Hants, and Dorset.
In Western France, the Alps, the Carpathians, Northern Italy, and the Apennines, an extensive series of rocks has been described by Continental geologists under the name of Tithonian. These beds, which are without any marine equivalent in this country, appear completely to bridge over the interval between the Neocomian and the Oolites. They may, perhaps, as suggested by Mr. Judd, be of the same age as part of the Wealden series.
UPPER NEOCOMIAN.
FOLKSTONE AND HYTHE BEDS.
(FIGURE 277. Nautilus plicatus, Sowerby, in Fitton’s Monog.)
(FIGURE 278. Ancyloceras gigas, d’Orbigny.)
(FIGURE 279. Gervillia anceps, Desh. Upper Neocomian, Surrey.)
(FIGURE 280. Trigonia caudata, Agassiz. Upper Neocomian.)
(FIGURE 281. Terebratula sella, Sowerby. Upper Neocomian, Hythe.)
(FIGURE 282. Diceras Lonsdalii. Upper Neocomian, Wilts. a. The bivalve shell.
b. Cast of one of the valves enlarged.)
The sands which crop out beneath the Gault in Wiltshire, Surrey, and Sussex are sometimes in the uppermost part pure white, at others of a yellow and ferruginous colour, and some of the beds contain much green matter. At Folkestone they contain layers of calcareous matter and chert, and at Hythe, in the neighbourhood, as also at Maidstone and other parts of Kent, the limestone called Kentish Rag is intercalated. This somewhat clayey and calcareous stone forms strata two feet thick, alternating with quartzose sand. The total thickness of these Folkestone and Hythe beds is less than 300 feet, and they are seen to rest immediately on a grey clay, to which we shall presently allude as the Atherfield clay. Among the fossils of the Folkestone and Hythe beds we may mention Nautilus plicatus (Figure 277), Ancyloceras (Scaphites) gigas (Figure 278), which has been aptly described as an Ammonite more or less uncoiled; Trigonia caudata (Figure 280), Gervillia anceps (Figure 279), a bivalve genus allied to Avicula, and Terebratula sella (Figure 281). In ferruginous beds of the same age in Wiltshire is found a remarkable shell called Diceras Lonsdalii (Figure 282), which abounds in the Upper and Middle Neocomian of Southern Europe. This genus is closely allied to Chama, and the cast of the interior has been compared to the horns of a goat.
ATHERFIELD CLAY.
We mentioned before that the Folkstone and Hythe series rest on a grey clay. This clay is only of slight thickness in Kent and Surrey, but acquires great dimensions at Atherfield, in the Isle of Wight. The difference, indeed, in mineral character and thickness of the Upper Neocomian formation near Folkestone, and the corresponding beds in the south of the Isle of Wight, about 100 miles distant, is truly remarkable. In the latter place we find no limestone answering to the Kentish Rag, and the entire thickness from the bottom of the Atherfield clay to the top of the Neocomian, instead of being less than 300 feet as in Kent, is given by the late Professor E. Forbes as 843 feet, which he divides into sixty-three strata, forming three groups. The uppermost of these consists of ferruginous sands, the second of sands and clay, and the third or lowest of a brown clay, abounding in fossils.
Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains of chlorite and mica, and, as Mr. Godwin-Austen has shown, fragments and water- worn fossils of the oolitic rocks, speak plainly of the nature of the pre- existing formations, by the wearing down of which the Neocomian beds were formed. The land, consisting of such rocks, was doubtless submerged before the origin of the white chalk, a deposit which was formed in a more open sea, and in clearer waters.
(FIGURE 283. Perna Mulleti, Desh. One-eighth natural size. a. Exterior.
b. Part of hinge-line of upper or right valve.)
Among the shells of the Atherfield clay the biggest and most abundant shell is the large Perna Mulleti, of which a reduced figure is given in Figure 283.
SIMILARITY OF CONDITIONS CAUSING REAPPEARANCE OF SPECIES.
Some species of mollusca and other fossils range through the whole series, while others are confined to particular subdivisions, and Forbes laid down a law which has since been found of very general application in regard to estimating the chronological relations of consecutive strata. Whenever similar conditions, he says, are repeated, the same species reappear, provided too great a lapse of time has not intervened; whereas if the length of the interval has been geologically great, the same genera will reappear represented by distinct species. Changes of depth, or of the mineral nature of the sea-bottom, the presence or absence of lime or of peroxide of iron, the occurrence of a muddy, or a sandy, or a gravelly bottom, are marked by the banishment of certain species and the predominance of others. But these differences of conditions being mineral, chemical, and local in their nature, have no necessary connection with the extinction, throughout a large area, of certain animals or plants. When the forms proper to loose sand or soft clay, or to perfectly clear water, or to a sea of moderate or great depth, recur with all the same species, we may infer that the interval of time has been, geologically speaking, small, however dense the mass of matter accumulated. But if, the genera remaining the same, the species are changed, we have entered upon a new period; and no similarity of climate, or of geographical and local conditions, can then recall the old species which a long series of destructive causes in the animate and inanimate world has gradually annihilated.
SPEETON CLAY, UPPER DIVISION.
(FIGURE 284. Ammonites Deshayesii, Leym. Upper Neocomian.)
On the coast, beneath the white chalk of Flamborough Head, in Yorkshire, an argillaceous formation crops out, called the Speeton clay, several hundred feet in thickness, the palaeontological relations of which have been ably worked out by Mr. John W. Judd, and he has shown that it is separable into three divisions, the uppermost of which, 150 feet thick, and containing 87 species of mollusca, decidedly belongs to the Atherfield clay and associated strata of Hythe and Folkestone, already described. (Judd, Speeton clay, Quarterly Geological Journal volume 24 1868 page 218.) It is characterised by the Perna Mulleti (Figure 283) and Terebratula sella (Figure 281), and by Ammonites Deshayesii (Figure 284), a well-known Hythe fossil. Fine skeletons of reptiles of the genera Pliosaurus and Teleosaurus have been obtained from this clay. At the base of this upper division of the Speeton clay there occurs a layer of large Septaria, formerly worked for the manufacture of cement. This bed is crowded with fossils, especially Ammonites, one species of which, three feet in diameter, was observed by Mr. Judd.
MIDDLE NEOCOMIAN.
TEALBY SERIES.
(FIGURE 285. Pecten cinctus, Sowerby. (P. crassitesta, Rom.) Middle Neocomian, England; Middle and Lower Neocomian, Germany. One-fifth natural size.)
(FIGURE 286. Ancyloceras (Crioceras) Duvallei, Leveille. Middle and Lower Neocomian. One-fifth natural size.)
At Tealby, a village in the Lincolnshire Wolds, there crop out beneath the white chalk some non-fossiliferous ferruginous sands about twenty-feet thick, beneath which are beds of clay and limestone, about fifty feet thick, with an interesting suite of fossils, among which are Pecten cinctus (Figure 285), from 9 to 12 inches in diameter, Ancyloceras Duvallei (Figure 286), and some forty other shells, many of them common to the Middle Speeton clay, about to be mentioned. Mr. Judd remarks that as Ammonites clypei-formis and Terebratula hippopus characterise the Middle Neocomian of the Continent, it is to this stage that the Tealby series containing the same fossils may be assigned. (Judd Quarterly Geological Journal 1867 volume 23 page 249.)
The middle division of the Speeton clay, occurring at Speeton below the cement- bed, before alluded to, is 150 feet thick, and contains about 39 species of mollusca, half of which are common to the overlying clay. Among the peculiar shells, Pecten cinctus (Figure 285) and Ancyloceras (Crioceras) Duvallei (Figure 286) occur.
LOWER NEOCOMIAN.
(FIGURE 287. Ammonites Noricus, Schloth. Lower Neocomian, Speeton.)
In the lower division of the Speeton clay, 200 feet thick, 46 species of mollusca have been found, and three divisions, each characterised by its peculiar ammonite, have been noticed by Mr. Judd. The central zone is marked by Ammonites Noricus (see Figure 287). On the Continent these beds are well-known by their corresponding fossils, the Hils clay and conglomerate of the north of Germany agreeing with the Middle and Lower Speeton, the latter of which, with the same mineral characters and fossils as in Yorkshire, is also found in the little island of Heligoland. Yellow limestone, which I have myself seen near Neuchatel, in Switzerland, represents the Lower Neocomian at Speeton.
WEALDEN FORMATION.
Beneath the Atherfield clay or Upper Neocomian of the S.E. of England, a fresh- water formation is found, called the Wealden, which, although it occupies a small horizontal area in Europe, as compared to the White Chalk and the marine Neocomian beds, is nevertheless of great geological interest, since the imbedded remains give us some insight into the nature of the terrestrial fauna and flora of the Lower Cretaceous epoch. The name of Wealden was given to this group because it was first studied in parts of Kent, Surrey, and Sussex, called the Weald; and we are indebted to Dr. Mantell for having shown, in 1822, in his “Geology of Sussex,” that the whole group was of fluviatile origin. In proof of this he called attention to the entire absence of Ammonites, Belemnites, Brachiopoda, Echinodermata, Corals, and other marine fossils, so characteristic of the Cretaceous rocks above, and of the Oolitic strata below, and to the presence in the Weald of Paludinae, Melaniae, Cyrenae, and various fluviatile shells, as well as the bones of terrestrial reptiles and the trunks and leaves of land-plants.
(FIGURE 288. Section from (left) W.S.W. through Brixton bay, Isle of Wight, Solent and South Downs to E.N.E. (right). 1. Tertiary.
2. Chalk and Gault.
3. Upper Neocomian (or Lower Greensand). 4. Wealden (Weald Clay and Hastings Sands).)
The evidence of so unexpected a fact as that of a dense mass of purely fresh- water origin underlying a deep-sea deposit (a phenomenon with which we have since become familiar) was received, at first, with no small doubt and incredulity. But the relative position of the beds is unequivocal; the Weald Clay being distinctly seen to pass beneath the Atherfield Clay in various parts of Surrey, Kent, and Sussex, and to reappear in the Isle of Wight at the base of the Cretaceous series, being, no doubt, continuous far beneath the surface, as indicated by the dotted lines in Figure 288. They are also found occupying the same relative position below the chalk in the peninsula of Purbeck, Dorsetshire, where, as we shall see in the sequel, they repose on strata referable to the Upper Oolite.
WEALD CLAY.
The Upper division, or Weald Clay, is, in great part, of fresh-water origin, but in its highest portion contains beds of oysters and other marine shells which indicate fluvio-marine conditions. The uppermost beds are not only conformable, as Dr. Fitton observes, to the inferior strata of the overlying Neocomian, but of similar mineral composition. To explain this, we may suppose that, as the delta of a great river was tranquilly subsiding, so as to allow the sea to encroach upon the space previously occupied by fresh-water, the river still continued to carry down the same sediment into the sea. In confirmation of this view it may be stated that the remains of the Iguanodon Mantelli, a gigantic terrestrial reptile, very characteristic of the Wealden, has been discovered near Maidstone, in the overlying Kentish Rag, or marine limestone of the Upper Neocomian. Hence we may infer that some of the saurians which inhabited the country of the great river continued to live when part of the district had become submerged beneath the sea. Thus, in our own times, we may suppose the bones of large alligators to be frequently entombed in recent fresh-water strata in the delta of the Ganges. But if part of that delta should sink down so as to be covered by the sea, marine formations might begin to accumulate in the same space where fresh-water beds had previously been formed; and yet the Ganges might still pour down its turbid waters in the same direction, and carry seaward the carcasses of the same species of alligator, in which case their bones might be included in marine as well as in subjacent fresh-water strata.
(FIGURES 289 AND 290. Tooth of Iguanodon Mantelli.
(FIGURE 289. a, and b.)
(FIGURE 290. A. Partially worn tooth of young individual of the same. b. Crown of tooth in adult worn down. (Mantell.)))
The Iguanodon, first discovered by Dr. Mantell, was an herbivorous reptile, of which the teeth, though bearing a great analogy, in their general form and crenated edges (see Figure 289 a and b), to the modern Iguanas which now frequent the tropical woods of America and the West Indies, exhibit many important differences. It appears that they have often been worn by the process of mastication; whereas the existing herbivorous reptiles clip and gnaw off the vegetable productions on which they feed, but do not chew them. Their teeth frequently present an appearance of having been chipped off, but never, like the fossil teeth of the Iguanodon, have a flat ground surface (see Figure 290, b) resembling the grinders of herbivorous mammalia. Dr. Mantell computes that the teeth and bones of this species which passed under his examination during twenty years must have belonged to no less than seventy-one distinct individuals, varying in age and magnitude from the reptile just burst from the egg, to one of which the femur measured twenty-four inches in circumference. Yet, notwithstanding that the teeth were more numerous than any other bones, it is remarkable that it was not until the relics of all these individuals had been found, that a solitary example of part of a jaw-bone was obtained. Soon afterwards remains both of the upper and lower jaw were met with in the Hastings beds in Tilgate Forest, near Cuckfield. In the same sands at Hastings, Mr. Beckles found large tridactyle impressions which it is conjectured were made by the hind feet of this animal, on which it is ascertained that there were only three well-developed toes.
(FIGURE 291. Cypris spinigera, Fitton.)
(FIGURE 292. Weald clay with Cyprides.)
Occasionally bands of limestone, called Sussex Marble, occur in the Weald Clay, almost entirely composed of a species of Paludina, closely resembling the common P. vivipara of English rivers. Shells of the Cypris, a genus of Crustaceans mentioned in Chapter 3 as abounding in lakes and ponds, are also plentifully scattered through the clays of the Wealden, sometimes producing, like plates of mica, a thin lamination (see Figure 292).
HASTINGS SANDS.
This lower division of the Wealden consists of sand, sandstone, calciferous grit, clay, and shale; the argillaceous strata, notwithstanding the name, predominating somewhat over the arenaceous, as will be seen by reference to the following table, drawn up by Messrs. Drew and Foster, of the Geological Survey of Great Britain:
TABLE 18.1. SUBORDINATE FORMATIONS IN THE HASTINGS SAND.
COLUMN 1: NAME OF SUBORDINATE FORMATION. COLUMN 2: MINERAL COMPOSITION OF THE STRATA. COLUMN 3: THICKNESS IN FEET.
Tunbridge Wells Sand: Sandstone and loam: 150.
Wadhurst Clay: Blue and brown shale and clay, with a little calc-grit: 100.
Ashdown Sand: Hard sand, with some beds of calc-grit: 160.
Ashburnham Beds: Mottled white and red clay, with some sandstone: 330.
The picturesque scenery of the “High Rocks” and other places in the neighbourhood of Tunbridge Wells is caused by the steep natural cliffs, to which a hard bed of white sand, occurring in the upper part of the Tunbridge Wells Sand, mentioned in the above table, gives rise. This bed of “rock-sand” varies in thickness from 25 to 48 feet. Large masses of it, which were by no means hard or capable of making a good building-stone, form, nevertheless, projecting rocks with perpendicular faces, and resist the degrading action of the river because, says Mr. Drew, they present a solid mass without planes of division. The calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in which the remains of the Iguanodon and Hylaeosaurus were first found by Dr. Mantell, constitute an upper member of the Tunbridge Wells Sand, while the “sand-rock” of the Hastings cliffs, about 100 feet thick, is one of the lower members of the same. The reptiles, which are very abundant in this division, consist partly of saurians, referred by Owen and Mantell to eight genera, among which, besides those already enumerated, we find the Megalosaurus and Plesiosaurus. The Pterodactyl also, a flying reptile, is met with in the same strata, and many remains of Chelonians of the genera Trionyx and Emys, now confined to tropical regions.
(FIGURE 293. Lepidotus Mantelli, Agassiz. Wealden. a. Palate and teeth.
b. Side view of teeth.
c. Scale.)
The fishes of the Wealden are chiefly referable to the Ganoid and Placoid orders. Among them the teeth and scales of Lepidotus are most widely diffused (see Figure 293). These ganoids were allied to the Lepidosteus, or Gar-pike, of the American rivers. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part coated with enamel. Most of the species of this genus are supposed to have been either river-fish, or inhabitants of the sea at the mouth of estuaries.
(FIGURE 294. Unio Valdensis, Mant. Isle of Wight and Dorsetshire; in the lower beds of the Hastings Sands. a, b.)
(FIGURE 295. Underside of slab of sandstone about one yard in diameter. Stammerham, Sussex.)
At different heights in the Hastings Sands, we find again and again slabs of sandstone with a strong ripple-mark, and between these slabs beds of clay many yards thick. In some places, as at Stammerham, Horsham, near there, are indications of this clay having been exposed so as to dry and crack before the next layer was thrown down upon it. The open cracks in the clay have served as moulds, of which casts have been taken in relief, and which are, therefore, seen on the lower surface of the sandstone (see Figure 295).
(FIGURE 296. Sphenopteris gracilis, Fitton. From the Hastings Sands near Tunbridge Wells.
a. A portion of the same magnified.)
Near the same place a reddish sandstone occurs in which are innumerable traces of a fossil vegetable, apparently Sphenopteris, the stems and branches of which are disposed as if the plants were standing erect on the spot where they originally grew, the sand having been gently deposited upon and around them; and similar appearances have been remarked in other places in this formation. (Mantell Geology of S.E. of England page 244.) In the same division also of the Wealden, at Cuckfield, is a bed of gravel or conglomerate, consisting of water- worn pebbles of quartz and jasper, with rolled bones of reptiles. These must have been drifted by a current, probably in water of no great depth.
From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden, the whole of it was a deposit in water of a moderate depth, and often extremely shallow. This idea may seem startling at first, yet such would be the natural consequence of a gradual and continuous sinking of the ground in an estuary or bay, into which a great river discharged its turbid waters. By each foot of subsidence, the fundamental rock would be depressed one foot farther from the surface; but the bay would not be deepened, if newly- deposited mud and sand should raise the bottom one foot. On the contrary, such new strata of sand and mud might be frequently laid dry at low water, or overgrown for a season by a vegetation proper to marshes.
PUNFIELD BEDS, BRACKISH AND MARINE.
(FIGURE 297. Vicarya Lujani, De Verneuil (Foss. de Utrillas.) Wealden, Punfield. a. Nearly perfect shell.
b. Vertical section of smaller specimen, showing continuous ridges as in Nerinaea.)
The shells of the Wealden beds belong to the genera Melanopsis, Melania, Paludina, Cyrena, Cyclas, Unio (see Figure 294), and others, which inhabit rivers or lakes; but one band has been found at Punfield, in Dorsetshire, indicating a brackish state of the water, where the genera Corbula, Mytilus, and Ostrea occur; and in some places this bed becomes purely marine, containing some well-known Neocomian fossils, among which Ammonites Deshayesii (Figure 284) may be mentioned. Others are peculiar as British, but very characteristic of the Upper and Middle Neocomian of Spain, and among these the Vicarya Lujani (Figure 297), a shell allied to Nerinea, is conspicuous.
By reference to Table 18.1 it will be seen that the Wealden beds are given as the fresh-water equivalents of the Marine Neocomian. The highest part of them in England may, for reasons just given, be regarded as Upper Neocomian, while some of the inferior portions may correspond in age to the Middle and Lower divisions of that group. In favour of this latter view, M. Marcou mentions that a fish called Asteracanthus granulosus, occurring in the Tilgate beds, is characteristic of the lowest beds of the Neocomian of the Jura, and it is well known that Corbula alata, common in the Ashburnham beds, is found also at the base of the Neocomian of the Continent.
AREA OF THE WEALDEN.
In regard to the geographical extent of the Wealden, it can not be accurately laid down, because so much of it is concealed beneath the newer marine formations. It has been traced about 320 English miles from west to east, from the coast of Dorsetshire to near Boulogne, in France; and nearly 200 miles from north-west to south-east, from Surrey and Hampshire to Vassy, in France. If the formation be continuous throughout this space, which is very doubtful, it does not follow that the whole was contemporaneous; because, in all likelihood, the physical geography of the region underwent frequent changes throughout the whole period, and the estuary may have altered its form, and even shifted its place. Dr. Dunker, of Cassel, and H. von Meyer, in an excellent monograph on the Wealdens of Hanover and Westphalia, have shown that they correspond so closely, not only in their fossils, but also in their mineral characters, with the English series, that we can scarcely hesitate to refer the whole to one great delta. Even then, the magnitude of the deposit may not exceed that of many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into the interior for more than 170 miles, and occupies, it is supposed, a space of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one-half of England. (Fitton Geology of Hastings