Each dirt-bed may, no doubt, be the memorial of many thousand years or centuries, because we find that two or three feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade. Yet, even if we imagine the fossil soils of the Lower Purbeck to represent as many ages, we need not be surprised to find that they do not constitute lines of separation between strata characterised by different zoological types. The preservation of a layer of vegetable soil, when in the act of being submerged, must be regarded as a rare exception to a general rule. It is of so perishable a nature, that it must usually be carried away by the denuding waves or currents of the sea, or by a river; and many Purbeck dirt-beds were probably formed in succession and annihilated, besides those few which now remain.

The plants of the Purbeck beds, so far as our knowledge extends at present, consist chiefly of Ferns, Coniferae, and Cycadeae (Figure 308), without any angiosperms; the whole more allied to the Oolitic than to the Cretaceous vegetation. The same affinity is indicated by the vertebrate and invertebrate animals. Mr. Brodie has found the remains of beetles and several insects of the homopterous and trichopterous orders, some of which now live on plants, while others are of such forms as hover over the surface of our present rivers.

PORTLAND OOLITE AND SAND (b, TABLE 19.1).

(FIGURE 311. Cerithium Portlandicum (=Terebra) Sowerby. a. Cast of shell known as "Portland screw." b. The shell itself. )

(FIGURE 312. Isastraea oblonga, M. Edw. and J. Haime. As seen on a polished slab of chert from the Portland Sand, Tisbury.)

(FIGURE 313. Trigonia gibbosa. 1/2 natural size. Portland Stone, Tisbury. a. The hinge.)

(FIGURE 314. Cardium dissimile. 1/4 natural size. Portland Stone.)

(FIGURE 315. Ostrea expansa. Portland Sand.)

The Portland Oolite has already been mentioned as forming in Dorsetshire the foundation on which the fresh-water limestone of the Lower Purbeck reposes (see above). It supplies the well-known building-stone of which St. Paul's and so many of the principal edifices of London are constructed. About fifty species of mollusca occur in this formation, among which are some ammonites of large size. The cast of a spiral univalve called by the quarrymen the "Portland screw" (a, Figure 311), is common; the shell of the same (b) being rarely met with. Also Trigonia gibbosa (Figure 313) and Cardium dissimile (Figure 314). This upper member rests on a dense bed of sand, called the Portland Sand, containing similar marine fossils, below which is the Kimmeridge Clay. In England these Upper Oolite formations are almost wholly confined to the southern counties. But some fragments of them occur beneath the Neocomian or Speeton Clay on the coast of Yorkshire, containing many more fossils common to the Portlandian of the Continent than does the same formation in Dorsetshire. Corals are rare in this formation, although one species is found plentifully at Tisbury, Wiltshire, in the Portland Sand, converted into flint and chert, the original calcareous matter being replaced by silex (Figure 312).

KIMMERIDGE CLAY.

The Kimmeridge Clay consists, in great part, of a bituminous shale, sometimes forming an impure coal, several hundred feet in thickness. In some places in Wiltshire it much resembles peat; and the bituminous matter may have been, in part at least, derived from the decomposition of vegetables. But as impressions of plants are rare in these shales, which contain ammonites, oysters, and other marine shells, with skeletons of fish and saurians, the bitumen may perhaps be of animal origin. Some of the saurians (Pliosaurus) in Dorsetshire are among the most gigantic of their kind.

(FIGURE 316. Cardium striatulum. Kimmeridge Clay, Hartwell.)

(FIGURE 317. Ostrea deltoidea. Kimmeridge Clay, 1/4 natural size.)

(FIGURE 318. Gryphaea (Exogyra) virgula. Kimmeridge Clay.)

(FIGURE 319. Trigonellites latus, Park, Kimmeridge Clay.)

Among the fossils, amounting to nearly 100 species, may be mentioned Cardium striatulum (Figure 316) and Ostrea deltoidea (Figure 317), the latter found in the Kimmeridge Clay throughout England and the north of France, and also in Scotland, near Brora. The Gryphaea virgula (Figure 318), also met with in the Kimmeridge Clay near Oxford, is so abundant in the Upper Oolite of parts of France as to have caused the deposit to be termed "marnes a gryphees virgules." Near Clermont, in Argonne, a few leagues from St. Menehould, where these indurated marls crop out from beneath the Gault, I have seen them, on decomposing, leave the surface of every ploughed field literally strewed over with this fossil oyster. The Trigonellites latus (Aptychus of some authors)(Figure 319) is also widely dispersed through this clay. The real nature of the shell, of which there are many species in oolitic rocks, is still a matter of conjecture. Some are of opinion that the two plates have been the gizzard of a cephalopod; others, that it may have formed a bivalve operculum of the same.

SOLENHOFEN STONE.

(FIGURE 320. Skeleton of Pterodactylus crassirostris. Oolite of Pappenheim, near Solenhofen. a. This bone, consisting of four joints, is part of the fifth or outermost digit elongated, as in bats, for the support of a wing.)

The celebrated lithographic stone of Solenhofen in Bavaria, appears to be of intermediate age between the Kimmeridge clay and the Coral Rag, presently to be described. It affords a remarkable example of the variety of fossils which may be preserved under favourable circumstances, and what delicate impressions of the tender parts of certain animals and plants may be retained where the sediment is of extreme fineness. Although the number of testacea in this slate is small, and the plants few, and those all marine, count Munster had determined no less than 237 species of fossils when I saw his collection in 1833; and among them no less than seven SPECIES of flying reptiles or pterodactyls (see Figure 320), six saurians, three tortoises, sixty species of fish, forty-six of crustacea, and twenty-six of insects. These insects, among which is a libellula, or dragon-fly, must have been blown out to sea, probably from the same land to which the pterodactyls, and other contemporaneous air-breathers, resorted.

(FIGURE 321. Tail and feather of Archaeopteryx, from Solenhofen, and tail of living bird for comparison. A. Caudal vertebrae of Archaeopteryx macrura, Owen; with impression of tail- feathers; one-fifth natural size. B. Two caudal vertebrae of same; natural size. C. Single feather, found in 1861 at Solenhofen, by Von Meyer, and called Archaeopteryx lithographica; natural size. D. Tail of recent vulture (Gyps Bengalensis) showing attachment of tail-feathers in living birds; one-quarter natural size. E. Profile of caudal vertebrae of same; one-third natural size. e, e. Direction of tail-feathers when seen in profile. f. Ploughshare bone or broad terminal joint (seen also in f, D.))

In the same slate of Solenhofen a fine example was met with in 1862 of the skeleton of a bird almost entire, and retaining even its feathers so perfect that the vanes as well as the shaft are preserved. The head was at first supposed to be wanting, but Mr. Evans detected on the slab what seems to be the impression of the cranium and beak, much resembling in size and shape that of the jay or woodcock. This valuable specimen is now in the British Museum, and has been called by Professor Owen Archaeopteryx macrura. Although anatomists agree that it is a true bird, yet they also find that in the length of the bones of the tail, and some other minor points of its anatomy, it approaches more nearly to reptiles than any known living bird. In the living representatives of the class Aves, the tail-feathers are attached to a coccygian bone, consisting of several vertebrae united together, whereas in the Archaeopteryx the tail is composed of twenty vertebrae, each of which supports a pair of quill-feathers. The first five only of the vertebrae, as seen in A, have transverse processes, the fifteen remaining ones become gradually longer and more tapering. The feathers diverge outward from them at an angle of 45 degrees.

Professor Huxley in his late memoirs on the order of reptiles called Dinosaurians, which are largely represented in all the formations, from the Neocomian to the Trias inclusive, has shown that they present in their structure many remarkable affinities to birds. But a reptile about two feet long, called Compsognathus, lately found in the Stonesfield slate, makes a much greater approximation to the class Aves than any Dinosaur, and therefore forms a closer link between the classes Aves and Reptilia than does the Archaeopteryx.

It appears doubtful whether any species of British fossil, whether of the vertebrate or invertebrate class, is common to the Oolite and Chalk. But there is no similar break or discordance as we proceed downward, and pass from one to another of the several leading members of the Jurassic group, the Upper, Middle, and Lower Oolite, and the Lias, there being often a considerable proportion of the mollusca, sometimes as much as a fourth, common to such divisions as the Upper and Middle Oolite.

MIDDLE OOLITE.

CORAL RAG.

(FIGURE 322. Thecosmilia annularis, Milne Edwards and J. Haime. Coral Rag, Steeple Ashton.)

(FIGURE 323. Thamnastraea. Coral Rag. Steeple Ashton.)

(FIGURE 324. Ostrea gregaria, Coral Rag, Steeple Ashton.)

One of the limestones of the Middle Oolite has been called the "Coral Rag," because it consists, in part, of continuous beds of petrified corals, most of them retaining the position in which they grew at the bottom of the sea. In their forms they more frequently resemble the reef-building polyparia of the Pacific than do the corals of any other member of the Oolite. They belong chiefly to the genera Thecosmilia (Figure 322), Protoseris, and Thamnastraea, and sometimes form masses of coral fifteen feet thick. In Figure 323 of a Thamnastraea from this formation, it will be seen that the cup-shaped cavities are deepest on the right-hand side, and that they grow more and more shallow, until those on the left side are nearly filled up. The last-mentioned stars are supposed to represent a perfected condition, and the others an immature state. These coralline strata extend through the calcareous hills of the north-west of Berkshire, and north of Wilts, and again recur in Yorkshire, near Scarborough. The Ostrea gregarea (Figure 324) is very characteristic of the formation in England and on the Continent.

(FIGURE 325. Nerinaea Goodhallii, Fitton. Coral Rag, Weymouth. 1/4 natural size.)

One of the limestones of the Jura, referred to the age of the English Coral Rag, has been called "Nerinaean limestone" (Calcaire a Nerinees) by M. Thirria; Nerinaea being an extinct genus of univalve shells (Figure 325) much resembling the Cerithium in external form. Figure 325 shows the curious and continuous ridges on the columnella and whorls.

OXFORD CLAY.

(FIGURE 326. Belemnites hastatus. Oxford Clay.)

(FIGURE 327. Ammonites Jason, Reinecke. (Syn. A. Elizabethae, Pratt. Oxford Clay, Christian Malford, Wiltshire.)

(FIGURE 328. Belemnites Puzosianus, d'Orbigny. B. Owenii, Pierce. Oxford Clay, Christian Malford, Wiltshire. a. Section of the shell projecting from the phragmacone. b-c. External covering to the ink-bag and phragmacone. c, d. Osselet, or that portion commonly called the belemnite. e. Conical chambered body called the phragmacone. f. Position of ink-bag beneath the shelly covering.)

The coralline limestone, or "Coral Rag," above described, and the accompanying sandy beds, called "calcareous grits," of the Middle Oolite, rest on a thick bed of clay, called the "Oxford Clay," sometimes not less than 600 feet thick. In this there are no corals, but great abundance of cephalopoda, of the genera Ammonite and Belemnite (Figures 326 and 327). In some of the finely laminated clays ammonites are very perfect, although somewhat compressed, and are frequently found with the lateral lobe extended on each side of the opening of the mouth into a horn-like projection (Figure 327). These were discovered in the cuttings of the Great Western Railway, near Chippenham, in 1841, and have been described by Mr. Pratt (Annals of Natural History November 1841).

Similar elongated processes have been also observed to extend from the shells of some Belemnites discovered by Dr. Mantell in the same clay (see Figure 328), who, by the aid of this and other specimens, has been able to throw much light on the structure of singular extinct forms of cuttle-fish. (See Philosophical Transactions 1850 page 363; also Huxley Memoirs of Geological Survey 1864; Phillips Palaeontological Society.)

KELLOWAY ROCK.

The arenaceous limestone which passes under this name is generally grouped as a member of the Oxford clay, in which it forms, in the south-west of England, lenticular masses, 8 or 10 feet thick, containing at Kelloway, in Wiltshire, numerous casts of ammonites and other shells. But in Yorkshire this calcareo- arenaceous formation thickens to about 30 feet, and constitutes the lower part of the Middle Oolite, extending inland from Scarborough in a southerly direction. The number of mollusca which it contains is, according to Mr. Etheridge, 143, of which only 34, or 23 1/2 per cent, are common to the Oxford clay proper. Of the 52 Cephalopoda, 15 (namely 13 species of ammonite, the Ancyloceras Calloviense and one Belemnite) are common to the Oxford Clay, giving a proportion of nearly 30 per cent.

LOWER OOLITE.

CORNBRASH AND FOREST MARBLE.

The upper division of this series, which is more extensive than the preceding or Middle Oolite, is called in England the Cornbrash, as being a brashy, easily broken rock, good for corn land. It consists of clays and calcareous sandstones, which pass downward into the Forest Marble, an argillaceous limestone, abounding in marine fossils. In some places, as at Bradford, this limestone is replaced by a mass of clay. The sandstones of the Forest Marble of Wiltshire are often ripple-marked and filled with fragments of broken shells and pieces of drift- wood, having evidently been formed on a coast. Rippled slabs of fissile oolite are used for roofing, and have been traced over a broad band of country from Bradford in Wilts, to Tetbury in Gloucestershire. These calcareous tile-stones are separated from each other by thin seams of clay, which have been deposited upon them, and have taken their form, preserving the undulating ridges and furrows of the sand in such complete integrity, that the impressions of small footsteps, apparently of crustaceans, which walked over the soft wet sands, are still visible. In the same stone the claws of crabs, fragments of echini, and other signs of a neighbouring beach, are observed. (P. Scrope Proceedings of the Geological Society March 1831.)

GREAT (OR BATH) OOLITE.

(FIGURE 329. Eunomia radiata, Lamouroux. (Calamophyllia, Milne Edwards.) a. Section transverse to the tubes. b. Vertical section, showing the radiation of the tubes. c. Portion of interior of tubes magnified, showing striated surface.)

Although the name of Coral Rag has been appropriated, as we have seen, to a member of the Middle Oolite before described, some portions of the Lower Oolite are equally entitled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which the Eunomia radiata (Figure 329) is very conspicuous, single individuals forming masses several feet in diameter; and having probably required, like the large existing brain-coral (Meandrina) of the tropics, many centuries before their growth was completed.

(FIGURE 330. Apiocrinites rotundus, or Pear Encrinite; Miller. Fossil at Bradford, Wilts. a. Stem of Apiocrinites, and one of the articulations, natural size. b. Section at Bradford of Great Oolite and overlying clay, containing the fossil encrinites. (See text.) c. Three perfect individuals of Apiocrinites, represented as they grew on the surface of the Great Oolite. d. Body of the Apiocrinites rotundus. Half natural size.)

(FIGURE 331. Apiocrinus. a. Single plate of body of Apiocrinus, overgrown with serpulae and bryozoa. Natural size. Bradford Clay. b. Portion of the same magnified, showing the bryozoan Diastopora diluviana covering one of the serpulae.)

Different species of crinoids, or stone-lilies, are also common in the same rocks with corals; and, like them, must have enjoyed a firm bottom, where their base of attachment remained undisturbed for years (c, Figure 330). Such fossils, therefore, are almost confined to the limestones; but an exception occurs at Bradford, near Bath, where they are enveloped in clay sometimes 60 feet thick. In this case, however, it appears that the solid upper surface of the "Great Oolite" had supported, for a time, a thick submarine forest of these beautiful zoophytes, until the clear and still water was invaded by a current charged with mud, which threw down the stone-lilies, and broke most of their stems short off near the point of attachment. The stumps still remain in their original position; but the numerous articulations, once composing the stem, arms, and body of the encrinite, were scattered at random through the argillaceous deposit in which some now lie prostrate. These appearances are represented in the section b, Figure 330, where the darker strata represent the Bradford clay, which is however a formation of such local development that in many places it can not easily be separated from the clays of the overlying "forest-marble" and underlying "fuller's earth." The upper surface of the calcareous stone below is completely incrusted over with a continuous pavement, formed by the stony roots or attachments of the Crinoidea; and besides this evidence of the length of time they had lived on the spot, we find great numbers of single joints, or circular plates of the stem and body of the encrinite, covered over with serpulae. Now these serpulae could only have begun to grow after the death of some of the stone-lilies, parts of whose skeletons had been strewed over the floor of the ocean before the irruption of argillaceous mud. In some instances we find that, after the parasitic serpulae were full grown, they had become incrusted over with a bryozoan, called Diastopora diluviana (see b, Figure 331); and many generations of these molluscoids had succeeded each other in the pure water before they became fossil.

We may, therefore, perceive distinctly that, as the pines and cycadeous plants of the ancient "dirt-bed," or fossil forest, of the Lower Purbeck were killed by submergence under fresh water, and soon buried beneath muddy sediment, so an invasion of argillaceous matter put a sudden stop to the growth of the Bradford Encrinites, and led to their preservation in marine strata.

Such differences in the fossils as distinguish the calcareous and argillaceous deposits from each other, would be described by naturalists as arising out of a difference in the STATIONS of species; but besides these, there are variations in the fossils of the higher, middle, and lower part of the oolitic series, which must be ascribed to that great law of change in organic life by which distinct assemblages of species have been adapted, at successive geological periods, to the varying conditions of the habitable surface. In a single district it is difficult to decide how far the limitation of species to certain minor formations has been due to the local influence of STATIONS, or how far it has been caused by time or the law of variation above alluded to. But we recognise the reality of the last-mentioned influence, when we contrast the whole oolitic series of England with that of parts of the Jura, Alps, and other distant regions, where, although there is scarcely any lithological resemblance, yet some of the same fossils remain peculiar in each country to the Upper, Middle, and Lower Oolite formations respectively. Mr. Thurmann has shown how remarkably this fact holds true in the Bernese Jura, although the argillaceous divisions, so conspicuous in England, are feebly represented there, and some entirely wanting.

(FIGURE 332. Terebratula digona, Sowerby. Natural size. Bradford Clay.)

(FIGURE 333. Purpuroidea nodulata. One-fourth natural size. Great Oolite, Minchinhampton.)

(FIGURE 334. Cylindrites acutus. Sowb. Syn. Actaeon acutus. Great Oolite, Minchinhampton.)

(FIGURE 335. Patella rugosa, Sowerby. Great Oolite.)

(FIGURE 336. Nerita costulata, Desh. Great Oolite.)

(FIGURE 337. Rimula (Emarginula) clathrata, Sowerby. Great Oolite.)

The calcareous portion of the Great Oolite consists of several shelly limestones, one of which, called the Bath Oolite, is much celebrated as a building-stone. In parts of Gloucestershire, especially near Minchinhampton, the Great Oolite, says Mr. Lycett, "must have been deposited in a shallow sea, where strong currents prevailed, for there are frequent changes in the mineral character of the deposit, and some beds exhibit false stratification. In others, heaps of broken shells are mingled with pebbles of rocks foreign to the neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, and crabs' claws. The shelly strata, also, have occasionally suffered denudation, and the removed portions have been replaced by clay." In such shallow-water beds shells of the genera Patella, Nerita, Rimula, Cylindrites are common (see Figures 334 to 337); while cephalopods are rare, and instead of ammonites and belemnites, numerous genera of carnivorous trachelipods appear. Out of 224 species of univalves obtained from the Minchinhampton beds, Mr. Lycett found no less than 50 to be carnivorous. They belong principally to the genera Buccinum, Pleurotoma, Rostellaria, Murex, Purpuroidea (Figure 333), and Fusus, and exhibit a proportion of zoophagous species not very different from that which obtains in seas of the Recent period. These zoological results are curious and unexpected, since it was imagined that we might look in vain for the carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, and it was a received doctrine that they did not begin to appear in considerable numbers till the Eocene period, when those two great families of cephalopoda, the ammonites and belemnites, and a great number of other representatives of the same class of chambered shells, had become extinct.

STONESFIELD SLATE: MAMMALIA.

(FIGURE 338. Elytron of Buprestis? Stonesfield.)

The slate of Stonesfield has been shown by Mr. Lonsdale to lie at the base of the Great Oolite. (Proceedings of the Geological Society volume 1 page 414.) It is a slightly oolitic shelly limestone, forming large lenticular masses imbedded in sand only six feet thick, but very rich in organic remains. It contains some pebbles of a rock very similar to itself, and which may be portions of the deposit, broken up on a shore at low water or during storms, and redeposited. The remains of belemnites, trigoniae, and other marine shells, with fragments of wood, are common, and impressions of ferns, cycadeae, and other plants. Several insects, also, and, among the rest, the elytra or wing-covers of beetles, are perfectly preserved (see Figure 338), some of them approaching nearly to the genus Buprestis. The remains, also, of many genera of reptiles, such as Plesiosaur, Crocodile, and Pterodactyl, have been discovered in the same limestone.

But the remarkable fossils for which the Stonesfield slate is most celebrated are those referred to the mammiferous class. The student should be reminded that in all the rocks described in the preceding chapters as older than the Eocene, no bones of any land-quadruped, or of any cetacean, had been discovered until the Spalacotherium of the Purbeck beds came to light in 1854. Yet we have seen that terrestrial plants were not wanting in the Upper Cretaceous formation (see Chapter 17), and that in the Wealden there was evidence of fresh-water sediment on a large scale, containing various plants, and even ancient vegetable soils. We had also in the same Wealden many land-reptiles and winged insects, which render the absence of terrestrial quadrupeds the more striking. The want, however, of any bones of whales, seals, dolphins, and other aquatic mammalia, whether in the chalk or in the upper or middle oolite, is certainly still more remarkable.

These observations are made to prepare the reader to appreciate more justly the interest felt by every geologist in the discovery in the Stonesfield slate of no less than ten specimens of lower jaws of mammiferous quadrupeds, belonging to four different species and to three distinct genera, for which the names of Amphitherium, Phascolotherium, and Stereognathus have been adopted.

(FIGURE 339. Tupaia tana. Right ramus of lower jaw. Natural size. A recent insectivorous placental mammal, from Sumatra.)

(Figures 340 and 341. Part of lower jaw of Tupaia tana. Twice natural size.

(FIGURE 340. End view seen from behind, showing the very slight inflection of the angle at c.)

(FIGURE 341. Side view of same.))

(Figures 342 and 343. Part of lower jaw of Didelphys Azarae; recent, Brazil. Natural size.

(FIGURE 342. End view seen from behind, showing the inflection of the angle of the jaw, c, d.)

(FIGURE 343. Side view of same.))

(FIGURE 344. Amphitherium Prevostii, Cuvier sp. Stonesfield Slate. Syn. Thylacotherium Prevostii, Valenc. a. Coronoid process. b. Condyle. c. Angle of jaw. d. Double-fanged molars.)

(FIGURE 345. Amphitheriumm Broderipii, Owen. Natural size. Stonesfield Slate.)

It is now generally admitted that these or really the remains of mammalia (although it was at first suggested that they might be reptiles), and the only question open to controversy is limited to this point, whether the fossil mammalia found in the Lower Oolite of Oxfordshire ought to be referred to the marsupial quadrupeds, or to the ordinary placental series. Cuvier had long ago pointed out a peculiarity in the form of the angular process (c, Figures 342 and 343) of the lower jaw, as a character of the genus Didelphys; and Professor Owen has since confirmed the doctrine of its generality in the entire marsupial series. In all these pouched quadrupeds this process is turned inward, as at c, d, Figure 342, in the Brazilian opossum, whereas in the placental series, as at c, Figures 340 and 341, there is an almost entire absence of such inflection. The Tupaia Tana of Sumatra has been selected by Mr. Waterhouse for this illustration, because the jaws of that small insectivorous quadruped bear a great resemblance to those of the Stonesfield Amphitherium. By clearing away the matrix from the specimen of Amphitherium Prevostii here represented (Figure 344), Professor Owen ascertained that the angular process (c) bent inward in a slighter degree than in any of the known marsupialia; in short, the inflection does not exceed that of the mole or hedgehog. This fact made him doubt whether the Amphitherium might not be an insectivorous placental, although it offered some points of approximation in its osteology to the marsupials, especially to the Myrmecobius, a small insectivorous quadruped of Australia, which has nine molars on each side of the lower jaw, besides a canine and three incisors. (A figure of this recent Myrmecobius will be found in my Principles of Geology chapter 9.) Another species of Amphitherium has been found at Stonesfield (Figure 345), which differs from the former (Figure 344) principally in being larger.

(FIGURE 346. Phascolotherium Bucklandi, Broderip, sp. a. Natural size. b. Molar of same, magnified.)

The second mammiferous genus discovered in the same slates was named originally by Mr. Broderip Didelphys Bucklandi (see Figure 346), and has since been called Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials in the general form of the jaw, and in the extent and position of its inflected angle, while the agreement with the living genus Didelphys in the number of the pre-molar and molar teeth is complete. (Owen's British Fossil Mammals page 62.)

In 1854 the remains of another mammifer, small in size, but larger than any of those previously known, was brought to light. The generic name of Stereognathus was given to it, and, as is usually the case in these old rocks (see above), it consisted of part of a lower jaw, in which were implanted three double-fanged teeth, differing in structure from those of all other known recent or extinct mammals.

PLANTS OF THE OOLITE.

(FIGURE 347. Portion of a fossil fruit of Podocarya Bucklandi, Ung., magnified. (Buckland's Bridgewater Treatise Plate 63.) Inferior Oolite, Charmouth, Dorset.)

(FIGURE 348. Cone of fossil Araucaria sphaerocarpa, Carruthers. Inferior Oolite. Bruton, Somersetshire. One-third diameter of original. In the collection of the British Museum.)

The Araucarian pines, which are now abundant in Australia and its islands, together with marsupial quadrupeds, are found in like manner to have accompanied the marsupials in Europe during the Oolitic period (see Figure 348). In the same rock endogens of the most perfect structure are met with, as, for example, fruits allied to the Pandanus, such as the Kaidacarpum ooliticum of Carruthers in the Great Oolite, and the Podocarya of Buckland (see Figure 347) in the Inferior Oolite.

FULLER'S EARTH.

(FIGURE 349. Ostrea acuminata. Fuller's Earth.)

Between the Great and Inferior Oolite near Bath, an argillaceous deposit, called "the fuller's earth," occurs; but it is wanting in the north of England. It abounds in the small oyster represented in Figure 349. The number of mollusca known in this deposit is about seventy; namely, fifty Lamellibranchiate Bivalves, ten Brachiopods, three Gasteropods, and seven or eight Cephalopods.

INFERIOR OOLITE.

This formation consists of a calcareous freestone, usually of small thickness, but attaining in some places, as in the typical area of Cheltenham and the Western Cotswolds, a thickness of 250 feet. It sometimes rests upon yellow sands, formerly classed as the sands of the Inferior Oolite, but now regarded as a member of the Upper Lias. These sands repose upon the Upper Lias clays in the south and west of England. The Collyweston slate, formerly classed with the Great Oolite, and supposed to represent in Northamptonshire the Stonesfield slate, is now found to belong to the Inferior Oolite, both by community of species and position in the series. The Collyweston beds, on the whole, assume a much more marine character than the Stonesfield slate. Nevertheless, one of the fossil plants Aroides Stutterdi, Carruthers, remarkable, like the Pandanaceous species before mentioned (Figure 347) as a representative of the monocotyledonous class, is common to the Stonesfield beds in Oxfordshire.

(FIGURE 350. Hemitelites Brownii, Goepp. Syn. Phlebopteris contigua, Lind. and Hutt. Lower carbonaceous strata, Inferior Oolite shales. Gristhorpe, Yorkshire.)

The Inferior Oolite of Yorkshire consists largely of shales and sandstones, which assume much the aspect of a true coal-field, thin seams of coal having actually been worked in them for more than a century. A rich harvest of fossil ferns has been obtained from them, as at Gristhorpe, near Scarborough (Figure 350). They contain also Cycadeae, of which family a magnificent specimen has been described by Mr. Williamson under the name Zamia gigas, and a fossil called Equisetum Columnare (see Figure 397), which maintains an upright position in sandstone strata over a wide area. Shells of Estheria and Unio, collected by Mr. Bean from these Yorkshire coal-bearing beds, point to the estuary or fluviatile origin of the deposit.

At Brora, in Sutherlandshire, a coal formation, probably coeval with the above, or at least belonging to some of the lower divisions of the Oolitic period, has been mined extensively for a century or more. It affords the thickest stratum of pure vegetable matter hitherto detected in any secondary rock in England. One seam of coal of good quality has been worked three and a half feet thick, and there are several feet more of pyritous coal resting upon it.

(FIGURE 351. Terebratula fimbria, Sowerby. Inferior Oolite marl. Cotswold Hills.)

(FIGURE 352. Rhynchonella spinosa, Schloth. Inferior Oolite.)

(FIGURE 353. Pholadomya fidicula, Sowerby. One-third natural size. Inferior Oolite.)

(FIGURE 354. Pleurotomaria granulata, Sowerby. Ferruginous Oolite, Normandy. Inferior Oolite, England.)

(FIGURE 355. Pleurotomaria ornata, Sowerby Sp. Inferior Oolite.)

(FIGURE 356. Collyrites (Dysaster) ringens, Agassiz. Inferior Oolite, Somersetshire.)

(FIGURE 357. Ammonites Humphresianus, Sowerby. Inferior Oolite.)

(FIGURE 358. Ammonites Braikenridgii, Sowerby. Oolite, Scarborough. Inferior Oolite, Dundry; Calvados; etc.)

(FIGURE 359. Ostrea Marshii. One-half natural size. Middle and Lower Oolite.)

Among the characteristic shells of the Inferior Oolite, I may instance Terebratula fimbria (Figure 351), Rhynchonella spinosa (Figure 352), and Pholadomya fidicula (Figure 353). The extinct genus Pleurotomaria is also a form very common in this division as well as in the Oolitic system generally. It resembles the Trochus in form, but is marked by a deep cleft (a, Figures 354, 355) on one side of the mouth. The Collyrites (Dysaster) ringens (Figure 356) is an Echinoderm common to the Inferior Oolite of England and France, as are the two Ammonites (Figures 357, 358).

PALAEONTOLOGICAL RELATIONS OF THE OOLITIC STRATA.

Observations have already been made on the distinctness of the organic remains of the Oolitic and Cretaceous strata, and the proportion of species common to the different members of the Oolite. Between the Lower Oolite and the Lias there is a somewhat greater break, for out of 256 mollusca of the Upper Lias, thirty- seven species only pass up into the Inferior Oolite.

In illustration of shells having a great vertical range, it may be stated that in England some few species pass up from the Lower to the Upper Oolite, as, for example, Rhynchonella obsoleta, Lithodomus inclusus, Pholadomya ovalis, and Trigonia costata.

(FIGURE 360. Ammonites macrocephalus, Schloth. One-third natural size. Great Oolite and Oxford Clay.)

Of all the Jurassic Ammonites of Great Britain, A. macrocephalus (Figure 360), which is common to the Great Oolite and Oxford Clay, has the widest range.

We have every reason to conclude that the gaps which occur, both between the larger and smaller sections of the English Oolites, imply intervals of time, elsewhere represented by fossiliferous strata, although no deposit may have taken place in the British area. This conclusion is warranted by the partial extent of many of the minor and some of the larger divisions even in England.

CHAPTER XX.

JURASSIC GROUP— CONTINUED.— LIAS.

Mineral Character of Lias. Numerous successive Zones in the Lias, marked by distinct Fossils, without Unconformity in the Stratification, or Change in the Mineral Character of the Deposits. Gryphite Limestone. Shells of the Lias. Fish of the Lias. Reptiles of the Lias. Ichthyosaur and Plesiosaur. Marine Reptile of the Galapagos Islands. Sudden Destruction and Burial of Fossil Animals in Lias. Fluvio-marine Beds in Gloucestershire, and Insect Limestone. Fossil Plants. The origin of the Oolite and Lias, and of alternating Calcareous and Argillaceous Formations.

LIAS.

The English provincial name of Lias has been very generally adopted for a formation of argillaceous limestone, marl, and clay, which forms the base of the Oolite, and is classed by many geologists as part of that group. The peculiar aspect which is most characteristic of the Lias in England, France, and Germany, is an alternation of thin beds of blue or grey limestone, having a surface which becomes light-brown when weathered, these beds being separated by dark-coloured, narrow argillaceous partings, so that the quarries of this rock, at a distance, assume a striped and ribbon-like appearance.

The Lias has been divided in England into three groups, the Upper, Middle, and Lower. The Upper Lias consists first of sands, which were formerly regarded as the base of the Oolite, but which, according to Dr. Wright, are by their fossils more properly referable to the Lias; secondly, of clay shale and thin beds of limestone. The Middle Lias, or marl-stone series, has been divided into three zones; and the Lower Lias, according to the labours of Quenstedt, Oppel, Strickland, Wright, and others, into seven zones, each marked by its own group of fossils. This Lower Lias averages from 600 to 900 feet in thickness.

From Devon and Dorsetshire to Yorkshire all these divisions, observes Professor Ramsay, are constant; and from top to bottom we can not assert that anywhere there is actual unconformity between any two subdivisions, whether of the larger or smaller kind.

In the whole of the English Lias there are at present known about 937 species of mollusca, and of these 267 are Cephalopods, of which class more than two-thirds are Ammonites, the Nautilus and Belemnite also abounding. The whole series has been divided by zones characterised by particular Ammonites; for while other families of shells pass from one division to another in numbers varying from about 20 to 50 per cent, these cephalopods are almost always limited to single zones, as Quenstedt and Oppel have shown for Germany, and Dr. Wright and others for England.

As no actual unconformity is known from the top of the Upper to the bottom of the Lower Lias, and as there is a marked uniformity in the mineral character of almost all the strata, it is somewhat difficult to account even for such partial breaks as have been alluded to in the succession of species, if we reject the hypothesis that the old species were in each case destroyed at the close of the deposition of the rocks containing them, and replaced by the creation of new forms when the succeeding formation began. I agree with Professor Ramsay in not accepting this hypothesis. No doubt some of the old species occasionally died out, and left no representatives in Europe or elsewhere; others were locally exterminated in the struggle for life by species which invaded their ancient domain, or by varieties better fitted for a new state of things. Pauses also of vast duration may have occurred in the deposition of strata, allowing time for the modification of organic life throughout the globe, slowly brought about by variation accompanied by extinction of the original forms.

FOSSILS OF THE LIAS.

(FIGURE 361. Plagiostoma (Lima) giganteum, Sowerby. Inferior Oolite and Lias.)

(FIGURE 362. Gryphaea incurva, Sowerby. (G. arcuata, Lam.) Lias.)

(FIGURE 363. Avicula inaequivalvis, Sowerby. Lower Lias.)

(FIGURE 364. Avicula cygnipes, Phil. Lower Lias, Gloucestershire and Yorkshire. a. Lower valve. b. Upper valve.)

(FIGURE 365. Hippopodium ponderosum, Sowerby. 1/4 diameter. Lias, Cheltenham)

(FIGURE 366. Spiriferina (Spirifera) Walcotti, Sowerby. Lower Lias.)

(FIGURE 367. Leptaena Moorei, Davidson. Upper Lias, Ilminster.)

The name of Gryphite limestone has sometimes been applied to the Lias, in consequence of the great number of shells which it contains of a species of oyster, or Gryphaea (Figure 362). A large heavy shell called Hippopodium (Figure 365), allied to Cypricardia, is also characteristic of the upper part of the Lower Lias. In this formation occur also the Aviculas, Figures 363 and 364. The Lias formation is also remarkable for being the newest of the secondary rocks in which brachiopoda of the genera Spirifer and Leptaena (Figures 366, 367) occur, although the former is slightly modified in structure so as to constitute the subgenus Spiriferina, Davidson, and the Leptaena has dwindled to a shell smaller in size than a pea. No less than eight or nine species of Spiriferina are enumerated by Mr. Davidson as belonging to the Lias. Palliobranchiate mollusca predominate greatly in strata older than the Trias; but, so far as we yet know, they did not survive the Liassic epoch.

(FIGURE 368. Ammonites Bucklandi, Sowerby. Ammonites bisulcatus, Brug. One- eighth diameter of original. a. Side view. b. Front view, showing mouth and bisulcated keel. Characteristic of the lower part of the Lias of England and the Continent.)

(FIGURE 369. Ammonites planorbis, Sowerby. One-half diameter of original. From the base of the Lower Lias of England and the Continent.)

(FIGURE 370. Nautilus truncatus, Sowerby. Lias.)

(FIGURE 371. Ammonites bifrons, Brug. Ammonites Walcotti, Sowerby. Upper Lias shales.)

(FIGURE 372. Ammonites margaritatus, Montf. Syn. Ammonites Stokesi, Sowerby. Middle Lias.)

Allusion has already been made to numerous zones in the Lias having each their peculiar Ammonites. Two of these occur near the base of the Lower Lias, having a united thickness, varying from 40 to 80 feet. The upper of these is characterised by Ammonites Bucklandi, and the lower by Ammonites planorbis (see Figures 368, 369). (Quarterly Journal volume 16 page 376.) Sometimes, however, there is a third intermediate zone, that of Ammonites angulatus, which is the equivalent of the zone called the infra-lias on the Continent, the species of which are for the most part common to the superior group marked by Ammonites Bucklandi.

(FIGURE 373. Extracrinus (Pentacrinus) Briareus. Miller. 1/2 natural size. (Body, arms, and part of stem.) Lower Lias, Lyme Regis.)

(FIGURE 374. Palaeocoma (Ophioderma) tenuibrachiata. E. Forbes. Middle Lias, Seatown, Dorset.)

Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are conspicuous. (See Figure 373.) Of Palaeocoma (Ophioderma) Egertoni (Figure 374), referable to the Ophiuridae of Muller, perfect specimens have been met with in the Middle Lias beds of Dorset and Yorkshire.

The Extracrinus Briareus (removed by Major Austin from Pentacrinus on account of generic differences) occurs in tangled masses, forming thin beds of considerable extent, in the Lower Lias of Dorset, Gloucestershire, and Yorkshire. The remains are often highly charged with pyrites. This Crinoid, with its innumerable tentacular arms, appears to have been frequently attached to the driftwood of the liassic sea, in the same manner as Barnacles float about on wood at the present day. There is another species of Extracrinus and several of Pentacrinus in the Lias; and the latter genus is found in nearly all the formations from the Lias to the London Clay inclusive. It is represented in the present seas by the delicate and rare Pentacrinus caput-medusae of the Antilles, which, with Comatula, is one of the few surviving members of the ancient family of the Crinoids, represented by so many extinct genera in the older formations.

FISHES OF THE LIAS.

(FIGURE 375. Scales of Lepidotus gigas. Agass. a. Two of the scales detached.)

(FIGURE 376. Aechmodus Leachii and Dapedius monilifer. a. Aechmodus. Restored outline. b. Scales of Aechmodus Leachii. c. Scales of Dapedius monilifer.)

(FIGURE 377. Acrodus nobilis, Agassiz (tooth); commonly called "fossil leech." Lias, Lyme Regis, and Germany.)

The fossil fish, of which there are no less than 117 species known as British, resemble generically those of the Oolite, but differ, according to M. Agassiz, from those of the Cretaceous period. Among them is a species of Lepidotus (L. gigas, Agassiz), Figure 375, which is found in the Lias of England, France, and Germany. (Agassiz Poissons Fossiles volume 2 tab. 28, 29.) This genus was before mentioned (Chapter 18) as occurring in the Wealden, and is supposed to have frequented both rivers and sea-coasts. Another genus of Ganoids (or fish with hard, shining, and enamelled scales), called Aechmodus (Figure 376), is almost exclusively Liassic. The teeth of a species of Acrodus, also, are very abundant in the Lias (Figure 377).

(FIGURE 378. Hybodus reticulatus, Agassiz. Lias, Lyme Regis. a. Part of fin, commonly called Ichthyodorulite. b. Tooth.)

(FIGURE 379. Chimaera monstrosa. (Agassiz Poissons Fossiles volume 3 tab. C Figure 1.) a. Spine forming anterior part of the dorsal fin.)

SBut the remains of fish which have excited more attention than any others are those large bony spines called ichthyodorulites (a, Figure 378), which were once supposed by some naturalists to be jaws, and by others weapons, resembling those of the living Balistes and Silurus; but which M. Agassiz has shown to be neither the one nor the other. The spines, in the genera last mentioned, articulate with the backbone, whereas there are no signs of any such articulation in the ichthyodorulites. These last appear to have been bony spines which formed the anterior part of the dorsal fin, like that of the living genera Cestracion and Chimaera (see a, Figure 379). In both of these genera, the posterior concave face is armed with small spines, as in that of the fossil Hybodus (Figure 378), a placoid fish of the shark family found fossil at Lyme Regis. Such spines are simply imbedded in the flesh, and attached to strong muscles. "They serve," says Dr. Buckland, "as in the Chimaera (Figure 379), to raise and depress the fin, their action resembling that of a movable mast, raising and lowering backward the sail of a barge." (Bridgewater Treatise page 290.)

REPTILES OF THE LIAS.

(FIGURE 380. Skeleton of Ichthyosaurus communis, restored by Conybeare and Cuvier. a. Costal vertebrae.)

(FIGURE 381. Skeleton of Plesiosaurus dolichodeirus, restored by Reverend W.D. Conybeare. a. Cervical vertebra.)

It is not, however, the fossil fish which form the most striking feature in the organic remains of the Lias; but the Enaliosaurian reptiles, which are extraordinary for their number, size, and structure. Among the most singular of these are several species of Ichthyosaurus and Plesiosaurus (Figures 380, 381). The genus Ichthyosaurus, or fish-lizard, is not confined to this formation, but has been found in strata as high as the White Chalk of England, and as low as the Trias of Germany, a formation which immediately succeeds the Lias in the descending order. It is evident from their fish-like vertebrae, their paddles, resembling those of a porpoise or whale, the length of their tail, and other parts of their structure, that the Ichthyosaurs were aquatic. Their jaws and teeth show that they were carnivorous; and the half-digested remains of fishes and reptiles, found within their skeletons, indicate the precise nature of their food.

Mr. Conybeare was enabled, in 1824, after examining many skeletons nearly perfect, to give an ideal restoration of the osteology of this genus, and of that of the Plesiosaurus (Geological Society Transactions Second Series volume 1 page 49.). (See Figures 380, 381.) The latter animal had an extremely long neck and small head, with teeth like those of the crocodile, and paddles analogous to those of the Ichthyosaurus, but larger. It is supposed to have lived in shallow seas and estuaries, and to have breathed air like the Ichthyosaur and our modern cetacea. (Conybeare and De la Beche, Geological Transactions First Series volume 5 page 559; and Buckland Bridgewater Treatise page 203.) Some of the reptiles above mentioned were of formidable dimensions. One specimen of Ichthyosaurus platydon, from the Lias at Lyme, now in the British Museum, must have belonged to an animal more than 24 feet in length; and there are species of Plesiosaurus which measure from 18 to 20 feet in length. The form of the Ichthyosaurus may have fitted it to cut through the waves like the porpoise; as it was furnished besides its paddles with a tail-fin so constructed as to be a powerful organ of motion; but it is supposed that the Plesiosaurus, at least the long-necked species (Figure 381), was better suited to fish in shallow creeks and bays defended from heavy breakers.

It is now very generally agreed that these extinct saurians must have inhabited the sea; and it was urged that as there are now chelonians, like the tortoise, living in fresh water, and others, as the turtle, frequenting the ocean, so there may have been formerly some saurians proper to salt, others to fresh water. The common crocodile of the Ganges is well-known to frequent equally that river and the brackish and salt water near its mouth; and crocodiles are said in like manner to be abundant both in the rivers of the Isla de Pinos (Isle of Pines), south of Cuba, and in the open sea round the coast. In 1835 a curious lizard (Amblyrhynchus cristatus) was discovered by Mr. Darwin in the Galapagos Islands. (See Darwin Naturalist's Voyage page 385 Murray.) It was found to be exclusively marine, swimming easily by means of its flattened tail, and subsisting chiefly on seaweed. One of them was sunk from the ship by a heavy weight, and on being drawn up after an hour was quite unharmed.

The families of Dinosauria, crocodiles, and Pterosauria or winged reptiles, are also represented in the Lias.

SUDDEN DESTRUCTION OF SAURIANS.

It has been remarked, and truly, that many of the fish and saurians, found fossil in the Lias, must have met with sudden death and immediate burial; and that the destructive operation, whatever may have been its nature, was often repeated.

"Sometimes," says Dr. Buckland, "scarcely a single bone or scale has been removed from the place it occupied during life; which could not have happened had the uncovered bodies of these saurians been left, even for a few hours, exposed to putrefaction, and to the attacks of fishes and other smaller animals at the bottom of the sea." (Bridgewater Treatise page 115.) Not only are the skeletons of the Ichthyosaurs entire, but sometimes the contents of their stomachs still remain between their ribs, as before remarked, so that we can discover the particular species of fish on which they lived, and the form of their excrements. Not unfrequently there are layers of these coprolites, at different depths in the Lias, at a distance from any entire skeletons of the marine lizards from which they were derived; "as if," says Sir H. De la Beche, "the muddy bottom of the sea received small sudden accessions of matter from time to time, covering up the coprolites and other exuviae which had accumulated during the intervals." (Geological Researches page 334.) It is further stated that, at Lyme Regis, those surfaces only of the coprolites which lay uppermost at the bottom of the sea have suffered partial decay, from the action of water before they were covered and protected by the muddy sediment that has afterwards permanently enveloped them.

Numerous specimens of the Calamary or pen-and-ink fish, (Geoteuthis bollensis) have also been met with in the Lias at Lyme, with the ink-bags still distended, containing the ink in a dried state, chiefly composed of carbon, and but slightly impregnated with carbonate of lime. These Cephalopoda, therefore, must, like the saurians, have been soon buried in sediment; for, if long exposed after death, the membrane containing the ink would have decayed. (Buckland Bridgewater Treatise page 307.)

As we know that river-fish are sometimes stifled, even in their own element, by muddy water during floods, it can not be doubted that the periodical discharge of large bodies of turbid fresh water in the sea may be still more fatal to marine tribes. In the "Principles of Geology" I have shown that large quantities of mud and drowned animals have been swept down into the sea by rivers during earthquakes, as in Java in 1699; and that indescribable multitudes of dead fishes have been seen floating on the sea after a discharge of noxious vapours during similar convulsions. But in the intervals between such catastrophes, strata may have accumulated slowly in the sea of the Lias, some being formed chiefly of one description of shell, such as ammonites, others of gryphites.

FRESH-WATER DEPOSITS.— INSECT BEDS.

(FIGURE 382. Wing of a neuropterous insect, from the Lower Lias, Gloucestershire. (Reverend P.B. Brodie.))

From the above remarks the reader will infer that the Lias is for the most part a marine deposit. Some members, however, of the series have an estuarine character, and must have been formed within the influence of rivers. At the base of the Upper and Lower Lias respectively, insect-beds appear to be almost everywhere present throughout the Midland and South-western districts of England. These beds are crowded with the remains of insects, small fish, and crustaceans, with occasional marine shells. One band in Gloucestershire, rarely exceeding a foot in thickness, has been named the "insect limestone." It passes upward, says the Reverend P.B. Brodie, into a shale containing Cypris and Estheria, and is full of the wing-cases of several genera of Coleoptera, with some nearly entire beetles, of which the eyes are preserved. (A History of Fossil Insects etc 1846 London.) The nervures of the wings of neuropterous insects (Figure 382) are beautifully perfect in this bed. Ferns, with Cycads and leaves of monocotyledonous plants, and some apparently brackish and fresh-water shells, accompany the insects in several places, while in others marine shells predominate, the fossils varying apparently as we examine the bed nearer or farther from the ancient land, or the source whence the fresh water was derived. After studying 300 specimens of these insects from the Lias, Mr. Westwood declares that they comprise both wood-eating and herb-devouring beetles, of the Linnean genera Elater, Carabus, etc., besides grasshoppers (Gryllus), and detached wings of dragon-flies and may-flies, or insects referable to the Linnean genera Libellula, Ephemera, Hemerobius, and Panorpa, in all belonging to no less than twenty-four families. The size of the species is usually small, and such as taken alone would imply a temperate climate; but many of the associated organic remains of other classes must lead to a different conclusion.

FOSSIL PLANTS.

Among the vegetable remains of the Lias, several species of Zamia have been found at Lyme Regis, and the remains of coniferous plants at Whitby. M. Ad. Brongniart enumerates forty-seven liassic acrogens, most of them ferns; and fifty gymnosperms, of which thirty-nine are cycads, and eleven conifers. Among the cycads the predominance of Zamites, and among the ferns the numerous genera with leaves having reticulated veins (as in Figure 349), are mentioned as botanical characteristics of this era. (Tableau des Veg. Foss. 1849 page 105.) The absence as yet from the Lias and Oolite of all signs of dicotyledonous angiosperms is worthy of notice. The leaves of such plants are frequent in tertiary strata, and occur in the Cretaceous, though less plentifully (see Chapter 17). The angiosperms seem, therefore, to have been at the least comparatively rare in these older secondary periods, when more space was occupied by the Cycads and Conifers.

ORIGIN OF THE OOLITE AND LIAS.

The entire group of Oolite and Lias consists of repeated alternations of clay, sandstone, and limestone, following each other in the same order. Thus the clays of the Lias are followed by the sands now considered (see Chapter 20) as belonging to the same formation, though formerly referred to the Inferior Oolite, and these sands again by the shelly and coralline limestone called the Great or Bath Oolite. So, in the Middle Oolite, the Oxford Clay is followed by calcareous grit and coral rag; lastly, in the Upper Oolite, the Kimmeridge Clay is followed by the Portland Sand and limestone (see Figure 298). (Conybeare and Philips's Outlines etc. page 166.) The clay beds, however, as Sir H. de la Beche remarks, can be followed over larger areas than the sand or sandstones. (Geological Researches page 337.) It should also be remembered that while the Oolite system becomes arenaceous and resembles a coal-field in Yorkshire, it assumes in the Alps an almost purely calcareous form, the sands and clays being omitted; and even in the intervening tracts it is more complicated and variable than appears in ordinary descriptions. Nevertheless, some of the clays and intervening limestones do retain, in reality, a pretty uniform character for distances of from 400 to 600 miles from east to west and north to south.

In order to account for such a succession of events, we may imagine, first, the bed of the ocean to be the receptacle for ages of fine argillaceous sediment, brought by oceanic currents, which may have communicated with rivers, or with part of the sea near a wasting coast. This mud ceases, at length, to be conveyed to the same region, either because the land which had previously suffered denudation is depressed and submerged, or because the current is deflected in another direction by the altered shape of the bed of the ocean and neighbouring dry land. By such changes the water becomes once more clear and fit for the growth of stony zoophytes. Calcareous sand is then formed from comminuted shell and coral, or, in some cases, arenaceous matter replaces the clay; because it commonly happens that the finer sediment, being first drifted farthest from coasts, is subsequently overspread by coarse sand, after the sea has grown shallower, or when the land, increasing in extent, whether by upheaval or by sediment filling up parts of the sea, has approached nearer to the spots first occupied by fine mud.

The increased thickness of the limestones in those regions, as in the Alps and Jura, where the clays are comparatively thin, arises from the calcareous matter having been derived from species of corals and other organic beings which live in clear water, far from land, to the growth of which the influx of mud would be unfavourable. Portions therefore of these clays and limestones have probably been formed contemporaneously to a greater extent than we can generally prove, for the distinctness of the species of organic beings would be caused by the difference of conditions between the more littoral and the more pelagic areas and the different depths and nature of the sea-bottom. Independently of those ascending and descending movements which have given rise to the superposition of the limestones and clays, and by which the position of land and sea are made in the course of ages to vary, the geologist has the difficult task of allowing for the contemporaneous thinning out in one direction and thickening in another, of the successive organic and inorganic deposits of the same era.

CHAPTER XXI.

TRIAS, OR NEW RED SANDSTONE GROUP.

Beds of Passage between the Lias and Trias, Rhaetic Beds. Triassic Mammifer. Triple Division of the Trias. Keuper, or Upper Trias of England. Reptiles of the Upper Trias. Foot-prints in the Bunter formation in England. Dolomitic Conglomerate of Bristol. Origin of Red Sandstone and Rock-salt. Precipitation of Salt from inland Lakes and Lagoons. Trias of Germany. Keuper. St. Cassian and Hallstadt Beds. Peculiarity of their Fauna. Muschelkalk and its Fossils. Trias of the United States. Fossil Foot-prints of Birds and Reptiles in the Valley of the Connecticut. Triassic Mammifer of North Carolina. Triassic Coal-field of Richmond, Virginia. Low Grade of early Mammals favourable to the Theory of Progressive Development.

BEDS OF PASSAGE BETWEEN THE LIAS AND TRIAS— RHAETIC BEDS.

We have mentioned in the last chapter that the base of the Lower Lias is characterised, both in England and Germany, by beds containing distinct species of Ammonites, the lowest subdivision having been called the zone of Ammonites planorbis. Below this zone, on the boundary line between the Lias and the strata of which we are about to treat, called "Trias," certain cream-coloured limestones devoid of fossils are usually found. These white beds were called by William Smith the White Lias, and they have been shown by Mr. Charles Moore to belong to a formation similar to one in the Rhaetian Alps of Bavaria, to which Mr. Gumbel has applied the name of Rhaetic. They have also long been known as the Koessen beds in Germany, and may be regarded as beds of passage between the Lias and Trias. They are named the Penarth beds by the Government surveyors of Great Britain, from Penarth, near Cardiff, in Glamorganshire, where they sometimes attain a thickness of fifty feet.

(FIGURE 383. Cardium rhaeticum, Merrian. Natural size. Rhaetic Beds.)

(FIGURE 384. Pecten Valoniensis. Dfr. 1/2 natural size. Portrush, Ireland, etc. Rhaetic Beds.)

(FIGURE 385. Avicula contorta. Portlock. Portrush, Ireland, etc. Natural size. Rhaetic Beds.)

The principal member of this group has been called by Dr. Wright the Avicula contorta bed, as this shell is very abundant, and has a wide range in Europe. (Dr. Wright, on Lias and Bone Bed, Quarterly Geological Journal 1860 volume 16.) General Portlock first described the formation as it occurs at Portrush, in Antrim, where the Avicula contorta is accompanied by Pecten Valoniensis, as in Germany.

The best known member of the group, a thin band or bone-breccia, is conspicuous among the black shales in the neighbourhood of Axmouth in Devonshire, and in the cliffs of Westbury-on-Severn, as well as at Aust and other places on the borders of the Bristol Channel. It abounds in the remains of saurians and fish, and was formerly classed as the lowest bed of the Lias; but Sir P. Egerton first pointed out, in 1841, that it should be referred to the Upper New Red Sandstone, because it contained an assemblage of fossil fish which are either peculiar to this stratum, or belong to species well-known in the Muschelkalk of Germany. These fish belong to the genera Acrodus, Hybodus, Gyrolepis, and Saurichthys.

(FIGURE 386. Hybodus plicatilis, Agassiz. Teeth. Bone-bed, Aust and Axmouth.)

(FIGURE 387. Saurichthys apicalis, Agassiz. Tooth; natural size and magnified. Axmouth.)

(FIGURE 388. Gyrolepis tenuistriatus, Agassiz. Scale; natural size and magnified. Axmouth.)

Among those common to the English bone-bed and the Muschelkalk of Germany are Hybodus plicatilis (Figure 386), Saurychthys apicalis (Figure 387), Gyrolepis tenuistriatus (Figure 388), and G. Albertii. Remains of saurians, Plesiosaurus among others, have also been found in the bone-bed, and plates of an Encrinus. It may be questioned whether some of those fossils which have the most Triassic character may not have been derived from the destruction of older strata, since in bone-beds, in general, many of the organic remains are undoubtedly derivative.

TRIASSIC MAMMIFER.

(FIGURE 389. Microlestes antiquus, Plieninger. Molar tooth, magnified. Upper Trias. Diegerloch, near Stuttgart, Wurtemberg. a. View of inner side? b. Same, outer side? c. Same in profile. d. Crown of same.)

In North-western Germany, as in England, there occurs beneath the Lias a remarkable bone breccia. It is filled with shells and with the remains of fishes and reptiles, almost all the genera of which, and some even of the species, agree with those of the subjacent Trias. This breccia has accordingly been considered by Professor Quenstedt, and other German geologists of high authority, as the newest or uppermost part of the Trias. Professor Plieninger found in it, in 1847, the molar tooth of a small Triassic mammifer, called by him Microlestes antiquus. He inferred its true nature from its double fangs, and from the form and number of the protuberances or cusps on the flat crown; and considering it as predaceous, probably insectivorous, he called it Microlestes from micros, little, and lestes, a beast of prey. Soon afterwards he found a second tooth, also at the same locality, Diegerloch, about two miles to the south-east of Stuttgart.

No anatomist had been able to give any feasible conjecture as to the affinities of this minute quadruped until Dr. Falconer, in 1857, recognised an unmistakable resemblance between its teeth and the two back molars of his new genus Plagiaulax (Figure 306), from the Purbeck strata. This would lead us to the conclusion that Microlestes was marsupial and plant-eating.

In Wurtemberg there are two bone-beds, namely, that containing the Microlestes, which has just been described, which constitutes, as we have seen, the uppermost member of the Trias, and another of still greater extent, and still more rich in the remains of fish and reptiles, which is of older date, intervening between the Keuper and Muschelkalk.

The genera Saurichthys, Hybodus, and Gyrolepis are found in both these breccias, and one of the species, Saurichthys Mongeoti, is common to both bone-beds, as is also a remarkable reptile called Nothosaurus mirabilis. The saurian called Belodon by H. von Meyer, of the Thecodont family, is another Triassic form, associated at Diegerloch with Microlestes.

TRIAS OF ENGLAND.

Between the Lias and the Coal (or Carboniferous group) there is interposed, in the midland and western counties of England, a great series of red loams, shales, and sandstones, to which the name of the "New Red Sandstone formation" was first given, to distinguish it from other shales and sandstones called the "Old Red," often identical in mineral character, which lie immediately beneath the coal. The name of "Red Marl" has been incorrectly applied to the red clays of this formation, as before explained (Chapter 2), for they are remarkably free from calcareous matter. The absence, indeed, of carbonate of lime, as well as the scarcity of organic remains, together with the bright red colour of most of the rocks of this group, causes a strong contrast between it and the Jurassic formations before described.

The group in question is more fully developed in Germany than in England or France. It has been called the Trias by German writers, or the Triple Group, because it is separable into three distinct formations, called the "Keuper," the "Muschelkalk," and the "Bunter-sandstein." Of these the middle division, or the Muschelkalk, is wholly wanting in England, and the uppermost (Keuper) and lowest (Bunter) members of the series are not rich in fossils.

UPPER TRIAS OR KEUPER.

In certain grey indurated marls below the bone-bed Mr. Boyd Dawkins has found at Watchet, on the coast of Somersetshire, a molar tooth of Microlestes, enabling him to refer to the Trias strata formerly supposed to be Liassic. Mr. Charles Moore had previously discovered many teeth of mammalia of the same family near Frome, in Somersetshire, in the contents of a vertical fissure traversing a mass of carboniferous limestone. The top of this fissure must have communicated with the bed of the Triassic sea, and probably at a point not far from the ancient shore on which the small marsupials of that era abounded.

This upper division of the Trias called the Keuper is of great thickness in the central counties of England, attaining, according to Mr. Hull's estimate, no less than 3450 feet in Cheshire, and it covers a large extent of country between Lancashire and Devonshire.

(FIGURE 390. Estheria minuta, Bronn.)

In Worcestershire and Warwickshire in sandstone belonging to the uppermost part of the Keuper the bivalve crustacean Estheria minuta occurs. The member of the English "New Red" containing this shell, in those parts of England, is, according to Sir Roderick Murchison and Mr. Strickland, 600 feet thick, and consists chiefly of red marl or slate, with a band of sandstone. Ichthyodorulites, or spines of Hybodus, teeth of fishes, and footprints of reptiles were observed by the same geologists in these strata.

(FIGURE 391. Hyperodapedon Gordoni. Left palate, maxillary. (Showing the two rows of palatal teeth on opposite sides of the jaw.) a. Under surface. b. Exterior right side.)

In the Upper Trias or Keuper the remains of two saurians of the order Lacertilia have been found. The one called Rhynchosaurus occurred at Grinsell near Shrewsbury, and is characterised by having a small bird-like skull and jaws without teeth. The other Hyperodapedon (Figure 391) was first noticed in 1858, near Elgin, in strata now recognised as Upper Triassic, and afterwards in beds of about the same age in the neighbourhood of Warwick. Remains of the same genus have been found both in Central India and Southern Africa in rocks believed to be of Triassic age. The Hyperodapedon has been shown by Professor Huxley to be a terrestrial reptile having numerous palatal teeth, and closely allied to the living Sphenodon of New Zealand.

The recent discoveries of a living saurian in New Zealand so closely allied to this supposed extinct division of the Lacertilia seems to afford an illustration of a principle pointed out by Mr. Darwin of the survival in insulated tracts, after many changes in physical geography, of orders of which the congeners have become extinct on continents where they have been exposed to the severer competition of a larger progressive fauna.

(FIGURE 392. Tooth of Labyrinthodon; natural size. Warwick sandstone.)

(FIGURE 393. Transverse section of upper part of tooth of Labyrinthodon Jaegeri, Owen (Mastodonsaurus Jaegeri, Meyer); natural size, and a segment magnified. a. Pulp cavity, from which the processes of pulp and dentine radiate.)

Teeth of Labyrinthodon (Figure 392) found in the Keuper in Warwickshire were examined microscopically by Professor Owen, and compared with other teeth from the German Keuper. He found after careful investigation that neither of them could be referred to true saurians, although they had been named Mastodonsaurus and Phytosaurus by Jager. It appeared that they were of the Batrachian order, and of gigantic dimensions in comparison with any representatives of that order now living. Both the Continental and English fossil teeth exhibited a most complicated texture, differing from that previously observed in any reptile, whether recent or extinct, but most nearly analogous to the Ichthyosaurus. A section of one of these teeth exhibits a series of irregular folds, resembling the labyrinthic windings of the surface of the brain; and from this character Professor Owen has proposed the name Labyrinthodon for the new genus. Figure 393 of part of one is given from his "Odontography," plate 64, a. The entire length of this tooth is supposed to have been about three inches and a half, and the breadth at the base one inch and a half.

ROCK-SALT.

In Cheshire and Lancashire there are red clays containing gypsum and salt of the age of the Trias which are between 1000 and 1500 feet thick. In some places lenticular masses of pure rock-salt nearly 100 feet thick are interpolated between the argillaceous beds. At the base of the formation beneath the rock- salt occur the Lower Sandstones and Marl, called provincially in Cheshire "water-stones," which are largely quarried for building. They are often ripple- marked, and are impressed with numerous footprints of reptiles.

The basement beds of the Keuper rest with a slight unconformability upon an eroded surface of the "Bunter" next to be described.

LOWER TRIAS OR BUNTER.

(FIGURE 394. Single footstep of Cheirotherium. Bunter-sandstein, Saxony, one- eighth of natural size.)

(FIGURE 395. Line of footsteps on slab of sandstone. Hildburghausen, in Saxony.)

The lower division or English representative of the "Bunter" attains a thickness of 1500 feet in the counties last mentioned, according to Professor Ramsay. Besides red and green shales and red sandstones, it comprises much soft white quartzose sandstone, in which the trunks of silicified trees have been met with at Allesley Hill, near Coventry. Several of them were a foot and a half in diameter, and some yards in length, decidedly of coniferous wood, and showing rings of annual growth. (Buckland Proceedings of the Geological Society volume 2 page 439 and Murchison and Strickland Geological Transactions Second Series volume 5 page 347.) Impressions, also, of the footsteps of animals have been detected in Lancashire and Cheshire in this formation. Some of the most remarkable occur a few miles from Liverpool, in the whitish quartzose sandstone of Storton Hill, on the west side of the Mersey. They bear a close resemblance to tracks first observed in this member of the Upper New Red Sandstone, at the village of Hesseberg, near Hildburghausen, in Saxony. For many years these footprints have been referred to a large unknown quadruped, provisionally named Cheirotherium by Professor Kaup, because the marks both of the fore and hind feet resembled impressions made by a human hand. (See Figure 394.) The foot- marks at Hesseberg are partly concave, and partly in relief, the former, or the depressions, are seen upon the upper surface of the sandstone slabs, but those in relief are only upon the lower surfaces, being, in fact, natural casts, formed in the subjacent footprints as in moulds. The larger impressions, which seem to be those of the hind foot, are generally eight inches in length, and five in width, and one was twelve inches long. Near each large footstep, and at a regular distance (about an inch and a half) before it, a smaller print of a fore foot, four inches long and three inches wide, occurs. The footsteps follow each other in pairs, each pair in the same line, at intervals of fourteen inches from pair to pair. The large as well as the small steps show the great toes alternately on the right and left side; each step makes the print of five toes, the first, or great toe, being bent inward like a thumb. Though the fore and hind foot differ so much in size, they are nearly similar in form.

As neither in Germany nor in England had any bones or teeth been met with in the same identical strata as the footsteps, anatomists indulged, for several years, in various conjectures respecting the mysterious animals from which they might have been derived. Professor Kaup suggested that the unknown quadruped might have been allied to the Marsupialia; for in the kangaroo the first toe of the fore foot is in a similar manner set obliquely to the others, like a thumb, and the disproportion between the fore and hind feet is also very great. But M. Link conceived that some of the four species of animals of which the tracks had been found in Saxony might have been gigantic Batrachians, and when it was afterwards inferred that the Labyrinthodon was an air-breathing reptile, it was conjectured by Professor Owen that it might be one and the same as the Cheirotherium.

DOLOMITIC CONGLOMERATE OF BRISTOL.

(FIGURE 396. Tooth of Thecodontosaurus; three times magnified. Riley and Stutchbury. Dolomitic conglomerate. Redland, near Bristol.)

Near Bristol, in Somersetshire, and in other counties bordering the Severn, the lowest strata belonging to the Triassic series consist of a conglomerate or breccia resting unconformably upon the Old Red Sandstone, and on different members of the Carboniferous rocks, such as the Coal Measures, Millstone Grit, and Mountain Limestone. This mode of superposition will be understood by reference to the section below Dundry Hill (Figure 85), where No. 4 is the dolomitic conglomerate. Such breccias may have been partly the result of the subaerial waste of an old land-surface which gradually sank down and suffered littoral denudation in proportion as it became submerged. The pebbles and fragments of older rocks which constitute the conglomerate are cemented together by a red or yellow base of dolomite, and in some places the encrinites and other fossils derived from the Mountain Limestone are so detached from the parent rocks that they have the deceptive appearance of belonging to a fauna contemporaneous with the dolomitic beds in which they occur. The imbedded fragments are both rounded and angular, some consisting of sandstone from the coal-measures, being of vast size, and weighing nearly a ton. Fractured bones and teeth of saurians which are truly of contemporaneous origin are dispersed through some parts of the breccia, and two of these reptiles called Thecodont saurians, named from the manner in which the teeth were implanted in the jawbone, obtained great celebrity because the patches of red conglomerate in which they were found, near Bristol, were originally supposed to be of Permian or Palaeozoic age, and therefore the only representatives in England of vertebrate animals of so high a grade in rocks of such antiquity. The teeth of these saurians are conical, compressed, and with finely serrated edges (see Figure 396); they are referred by Professor Huxley to the Dinosaurian order.

ORIGIN OF RED SANDSTONE AND ROCK-SALT.

In various parts of the world, red and mottled clays and sandstones, of several distinct geological epochs, are found associated with salt, gypsum, and magnesian limestone, or with one or all of these substances. There is, therefore, in all likelihood, a general cause for such a coincidence. Nevertheless, we must not forget that there are dense masses of red and variegated sandstones and clays, thousands of feet in thickness, and of vast horizontal extent, wholly devoid of saliferous or gypseous matter. There are also deposits of gypsum and of common salt, as in the blue-clay formation of Sicily, without any accompanying red sandstone or red clay.

These red deposits may be accounted for by the decomposition of gneiss and mica schist, which in the eastern Grampians of Scotland has produced a mass of detritus of precisely the same colour as the Old Red Sandstone.

It is a general fact, and one not yet accounted for, that scarcely any fossil remains are ever preserved in stratified rocks in which this oxide of iron abounds; and when we find fossils in the New or Old Red Sandstone in England, it is in the grey, and usually calcareous beds, that they occur. The saline or gypseous interstratified beds may have been produced by submarine gaseous emanations, or hot mineral springs, which often continue to flow in the same spots for ages. Beds of rock-salt are, however, more generally attributed to the evaporation of lakes or lagoons communicating at intervals with the ocean. In Cheshire two beds of salt occur of the extraordinary thickness of 90 or even 100 feet, and extending over an area supposed to be 150 miles in diameter. The adjacent beds present ripple-marked sandstones and footprints of animals at so many levels as to imply that the whole area underwent a slow and gradual depression during the formation of the red sandstone.

Major Harris, in his "Highlands of Ethiopia," describes a salt lake, called the Bahr Assal, near the Abyssinian frontier, which once formed the prolongation of the Gulf of Tadjara, but was afterwards cut off from the gulf by a broad bar of lava or of land upraised by an earthquake. "Fed by no rivers, and exposed in a burning climate to the unmitigated rays of the sun, it has shrunk into an elliptical basin, seven miles in its transverse axis, half filled with smooth water of the deepest caerulean hue, and half with a solid sheet of glittering snow-white salt, the offspring of evaporation." "If," says Mr. Hugh Miller, "we suppose, instead of a barrier of lava, that sand-bars were raised by the surf on a flat arenaceous coast during a slow and equable sinking of the surface, the waters of the outer gulf might occasionally topple over the bar, and supply fresh brine when the first stock had been exhausted by evaporation."

The Runn of Cutch, as I have shown elsewhere (Principles of Geology chapter 27.), is a low region near the delta of the Indus, equal in extent to about a quarter of Ireland, which is neither land nor sea, being dry during part of every year, and covered by salt water during the monsoons. Here and there its surface is incrusted over with a layer of salt caused by the evaporation of sea- water. A subsiding movement has been witnessed in this country during earthquakes, so that a great thickness of pure salt might result from a continuation of such sinking.

TRIAS OF GERMANY.

In Germany, as before hinted, chapter 21, the Trias first received its name as a Triple Group, consisting of two sandstones with an intermediate marine calcareous formation, which last is wanting in England.

NOMENCLATURE OF TRIAS.

COLUMN 1: GERMAN.

COLUMN 2: FRENCH.

COLUMN 3: ENGLISH.

Keuper: Marnes irisees: Saliferous and gypseous shales and sandstone.

Muschelkalk: Muschelkalk, ou calcaire coquillier: Wanting in England.

Bunter-sandstein: Gres bigarre: Sandstone and quartzose conglomerate.

KEUPER.

(FIGURE 397. Equisetites columnaris. (Syn. Equisetum columnare.) Fragment of stem, and a small portion of same magnified. Keuper.)

The first of these, or the Keuper, underlying the beds before described as Rhaetic, attains in Wurtemberg a thickness of about 1000 feet. It is divided by Alberti into sandstone, gypsum, and carbonaceous clay-slate. (Monog. des Bunter- Sandsteins.) Remains of reptiles called Nothosaurus and Phytosaurus, have been found in it with Labyrinthodon; the detached teeth, also, of placoid fish and of Rays, and of the genera Saurichthys and Gyrolepis (Figures 387, 388). The plants of the Keuper are generically very analogous to those of the oolite and lias, consisting of ferns, equisetaceous plants, cycads, and conifers, with a few doubtful monocotyledons. A few species such as Equisetites columnaris, are common to this group and the oolite.

ST. CASSIAN AND HALLSTADT BEDS (SEE MAP, FIGURE 398).

(FIGURE 398. Map of Tyrol and Styria showing St. Cassian and Hallstadt Beds.)

The sandstones and clay of the Keuper resemble the deposits of estuaries and a shallow sea near the land, and afford, in the north-west of Germany, as in France and England, but a scanty representation of the marine life of that period. We might, however, have anticipated, from its rich reptilian fauna, that the contemporaneous inhabitants of the sea of the Keuper period would be very numerous, should we ever have an opportunity of bringing their remains to light. This, it is believed, has at length been accomplished, by the position now assigned to certain Alpine rocks called the "St. Cassian beds," the true place of which in the series was until lately a subject of much doubt and discussion. It has been proved that the Hallstadt beds on the northern flanks of the Austrian Alps correspond in age with the St. Cassian beds on their southern declivity, and the Austrian geologists, M. Suess of Vienna and others, have satisfied themselves that the Hallstadt formation is referable to the period of the Upper Trias. Assuming this conclusion to be correct, we become acquainted suddenly and unexpectedly with a rich marine fauna belonging to a period previously believed to be very barren of organic remains, because in England, France, and Northern Germany the upper Trias is chiefly represented by beds of fresh or brackish water origin.

(FIGURE 399. Scoliostoma, St. Cassian.)

(FIGURE 400. Platystoma Suessii, Hornes. From Hallstadt.)

(FIGURE 401. Koninckia Leonhardi, Wissmann. a. Ventral view. Part of ventral valve removed to show the vascular impressions of dorsal valve. b. Interior of dorsal valve, showing spiral processes restored. c. Vertical section of both valves. Part shaded black showing place occupied by the animal, and the dorsal valve following the curve of the ventral.)

About 600 species of invertebrate fossils occur in the Hallstadt and St. Cassian beds, many of which are still undescribed; some of the Mollusca are of new and peculiar genera, as Scoliostoma, Figure 399, and Platystoma, Figure 400, among the Gasteropoda; and Koninckia, Figure 401, among the Brachiopoda.

TABLE 21.1 GENERA OF FOSSIL MOLLUSCA IN THE ST. CASSIAN AND HALLSTADT BEDS.

COLUMN 1: COMMON TO OLDER ROCKS. Orthoceras. Bactrites. Macrocheilus. Loxonema. Holopella. Murchisonia. Porcellia. Athyris. Retzia. Cyrtina. Euomphalus.

COLUMN 2: CHARACTERISTIC TRIASSIC GENERA. Ceratites. Cochloceras. Choristoceras. Rhabdoceras. Aulacoceras. Scoliostoma (reaches its maximum in the Trias, but passes down to older rocks). Naticella. Platystoma. Ptychostoma. Euchrysalis. Halobia. Hornesia. Amphiclina. Koninckia. Cassianella. (Reach their maximum in the Trias, but pass up to newer rocks.) Myophoria. (Reach their maximum in the Trias, but pass up to newer rocks.)

COLUMN 3: COMMON TO NEWER ROCKS. Ammonites. Chemnitzia. Cerithium. Monodonta. Opis. Sphoera. Cardita. Myoconcha. Hinnites. Monotis. Plicatula. Pachyrisma. Thecidium.

Table 21.1 of genera of marine shells from the Hallstadt and St. Cassian beds, drawn up first on the joint authority of M. Suess and the late Dr. Woodward, and since corrected by Messrs. Etheridge and Tate, shows how many connecting links between the fauna of primary and secondary Palaeozoic and Mesozoic rocks are supplied by the St. Cassian and Hallstadt beds.

The first column marks the last appearance of several genera which are characteristic of Palaeozoic strata. The second shows those genera which are characteristic of the Upper Trias, either as peculiar to it, or, as in the three cases marked by asterisks, reaching their maximum of development at this era. The third column marks the first appearance in Triassic rocks of genera destined to become more abundant in later ages.

It is only, however, when we contemplate the number of species by which each of the above-mentioned genera are represented that we comprehend the peculiarities of what is commonly called the St. Cassian fauna. Thus, for example, the Ammonite, which is not common to older rocks, is represented by no less than seventy-three species; whereas Loxonema, which is only known as common to older rocks, furnishes fifteen Triassic species. Cerithium, so abundant in tertiary strata, and which still lives, is represented by no less than fourteen species. As the Orthoceras had never been met with in the marine Muschelkalk, much surprise was naturally felt that seven or eight species of the genus should appear in the Hallstadt beds, assuming these last to belong to the Upper Trias. Among these species are some of large dimensions, associated with large Ammonites with foliated lobes, a form never seen before so low in the series, while the Orthoceras had never been seen so high.

On the whole, the rich marine fauna of Hallstadt and St. Cassian, now generally assigned to the lowest members of the Upper Trias or Keuper, leads us to suspect that when the strata of the Triassic age are better known, especially those belonging to the period of the Bunter sandstone, the break between the Palaeozoic and Mesozoic Periods may be almost effaced. Indeed some geologists are not yet satisfied that the true position of the St. Cassian beds (containing so great an admixture of types, having at once both Mesozoic and Palaeozoic affinities) is made out, and doubt whether they have yet been clearly proved to be newer than the Muschelkalk.

MUSCHELKALK.

(FIGURE 402. Ceratites nodosus, Schloth. Muschelkalk, Germany. Side and front view, showing the denticulated outline of the septa dividing the chambers.)

(FIGURE 403. Gervilia (Avicula) socialis, Schloth. Characteristic shell of the Muschelkalk.)

The next member of the Trias in Germany, the Muschelkalk, which underlies the Keuper before described, consists chiefly of a compact greyish limestone, but includes beds of dolomite in many places, together with gypsum and rock-salt. This limestone, a formation wholly unrepresented in England, abounds in fossil shells, as the name implies. Among the Cephalopoda there are no belemnites, and no ammonites with foliated sutures, as in the Lias, and Oolite, and the Hallstadt beds; but we find instead a genus allied to the Ammonite, called Ceratites by de Haan, in which the descending lobes (Figure 402) terminate in a few small denticulations pointing inward. Among the bivalve crustacea, the Estheria minuta, Bronn (see Figure 390), is abundant, ranging through the Keuper, Muschelkalk, and Bunter-sandstein; and Gervillia socialis (Figure 403), having a similar range, is found in great numbers in the Muschelkalk of Germany, France, and Poland.

(FIGURE 404. Encrinus liliiformis, Schlott. Syn. E. moniliformis. Body, arms, and part of stem. a. Section of stem. Muschelkalk.)

(FIGURE 405. Aspidura loricata, Agassiz. a. Upper side. b. Lower side. Muschelkalk.)

(FIGURE 406. Palatal teeth of Placodus gigas. Muschelkalk.)

The abundance of the heads and stems of lily encrinites, Encrinus liliiformis (Figure 404), (or Encrinites moniliformis), shows the slow manner in which some beds of this limestone have been formed in clear sea-water. The star-fish called Aspidura loricata (Figure 405) is as yet peculiar to the Muschelkalk. In the same formation are found the skull and teeth of a reptile of the genus Placodus (see Figure 406), which was referred originally by Munster, and afterwards by Agassiz, to the class of fishes. But more perfect specimens enabled Professor Owen, in 1858, to show that this fossil animal was a Saurian reptile, which probably fed on shell-bearing mollusks, and used its short and flat teeth, so thickly coated with enamel, for pounding and crushing the shells.

BUNTER-SANDSTEIN.

(FIGURE 407. Voltzia heterophylla. (Syn. Voltzia brevifolia.) b. Portion of same magnified to show fructification. Sulzbad. Bunter-sandstein.)

The Bunter-sandstein consists of various-coloured sandstones, dolomites, and red clays, with some beds, especially in the Hartz, of calcareous pisolite or roe- stone, the whole sometimes attaining a thickness of more than 1000 feet. The sandstone of the Vosges is proved, by its fossils, to belong to this lowest member of the Triassic group. At Sulzbad (or Soultz-les-bains), near Strasburg, on the flanks of the Vosges, many plants have been obtained from the "bunter," especially conifers of the extinct genus Voltzia, of which the fructification has been preserved. (See Figure 407.) Out of thirty species of ferns, cycads, conifers, and other plants, enumerated by M. Ad. Brongniart, in 1849, as coming from the "Gres bigarre," or Bunter, not one is common to the Keuper.