When these rocks were in the progress of formation, there existed the ichthyosaurus, or fish-lizard, which appears, in many respects, to have resembled the crocodile of the Nile. It was a predatory creature of enormous power, and must have been the tyrant and terror of the seas which it inhabited. Its alligator-like jaws, its powerful eye, its fish-like fins, and turtle-like paddles, were all formed to facilitate its progress as a destructive minister. The plesiosaurus was, if possible, a still more extraordinary creation. To the head of a lizard was united an enormously long neck, a small and fish-like body, and the tail of a crocodile: it appears formed for existence in shallow waters, so that, when moving at the bottom, it could lift its head above the surface for air, or in search of its food. The flora of this period must have been extensive; and it resembled the vegetation which exists at present in Tropical regions.
We pass now to a new epoch, which is well distinguished by its animals from all that had preceded it. Races of reptiles still have place upon the earth, and we have now the megalosaurian remains; these animals possessing a strength and rapacity which would render them objects of terror as well as astonishment, could they be restored to the world which they once ravaged. An enormous bat-like creature also existed at this time—the pterodactyl—which, in the language of Cuvier, was, “undoubtedly, the most extraordinary of all the beings of whose former existence a knowledge is granted to us, and that which, if seen alive, would appear most unlike anything that exists in the present world.” “You see before you,” says the same writer, “an animal which, in all points of bony structure, from the teeth to the extremities of the nails, presents the well-known saurian characteristics, and of which no one can doubt that its integuments and soft parts, its scaly armour and its organs of circulation and reproduction, are likewise analogous. But it was, at the same time, an animal provided with the means of flying; and, when stationary, its wings were probably folded back like those of a bird, although, perhaps, by the claws attached to its fingers, it might suspend itself from the branches of trees.”[235]
From the disintegration of the older rocks have no doubt arisen those formations which are known as the oolitic series. In these strata are preserved the remains of plants and animals more resembling those which now exist upon the earth; and, for the first time,—unless the evidence of the footsteps of birds on the new red sandstone of America be accepted,—we meet with the remains of the feathered tribes.
In these formations we discover animals belonging to the class Mammalia,—the amphitherium and the phascolotherium,—which appear to have resembled, in many respects, the marsupial animals of New Holland.[236]
The wealden formations, which are the next in order of position, are a series of clays and sands, with subordinate beds of limestone, grit, and shale. These have, in some instances, been formed in the sea; but they are usually regarded as fresh-water deposits. All the older rocks bear evident marks of marine origin, unless some of the coal-measure strata may be regarded as otherwise; but nearly all the wealden series contain the remains of land, fresh-water, and estuary animals, and of land vegetables. The creatures which we discover, preserved, to tell the history of this period, are numerous, and have marked peculiarities to distinguish them from those already described, or from any now existing on the earth. We find land saurians of a large kind, and animals of all sizes; even insects, of which a great variety are found in the wealds. The remarkable iguanodon was an animal which, even by the cautious measurement of Professor Owen, must have been at least twenty-eight feet long; and this enormous creature was suspected, by Cuvier, and has been proved by Owen, to have been an “herbivorous saurian for terrestrial life.”[237] Dr. Mantell calculates that no less than seventy individuals of the iguanodon of all ages have come under his notice; and the bones of a vast number of others must have been broken up by the workmen in the few quarries of Tilgate grit; so that these creatures were by no means rare at the period of their existence.[238]
The uppermost of these secondary formations is the cretaceous or chalk group, which spreads over a large portion of south-eastern England, and is met with in all parts of Europe. This chalk, which is a carbonate of lime, appears to have been slowly precipitated from tranquil water, as, according to Sir Henry De la Beche, organic remains are beautifully preserved in it. Substances of no greater solidity than common sponges retain their forms, delicate shells remain unbroken, fish even are frequently not flattened, and altogether we have the appearances which justify us in concluding that, since these organic exuviæ were entombed, they have been protected from pressure by the consolidation of the rock around them.[239]
Beneath the chalk exists what has been called, from its colour—derived from a silicate of the protoxide of iron,—green sand, and was, no doubt, formed by deposition from the same water in which the carbonate of lime was suspended,—the green sand falling to the bottom more readily from its greater specific gravity. “The tranquillity,” observes Sir Henry De la Beche, “which seems to have prevailed during this great accumulation of siliceo-calcareous matter, whether it may have been a deposit from water, in which it was mechanically suspended, partly the work of living creatures, or in a great measure chemical, is very remarkable.”[240]
In the chalk, the remains of the leaves of dicotyledonous plants and fragments of wood are found more abundantly than in the earlier strata, many of which are marked with the perforations of marine worms, indicating that they had floated for some time in the ocean. It should, however, be remembered, that these are not the first indications of vegetable life,—leaves have been found in the new red sandstone; and the flora of the coal formation must not be forgotten. The manner in which silica has deposited itself on organic bodies—such as the sponges—is curious; the whole of the organized tissue being often removed, and flint having taken its place. Flints formed by such a process as this abound in the upper chalk. The association of carbon and silicon, combined with oxygen, as we find them in the cretaceous formations, is most interesting, and naturally gives rise to some speculation on the relation of these two elements. Both carbon and silicon, as has been already shown, exist in several allotropic conditions; and, although the statements made by Dr. Brown relative to the conversion of carbon into silicon are proved to be grounded on experimental error, it is not improbable that a very intimate relation may exist between these elements.[241] The probability is, that the sponge animal has the power of secreting silica to give strength to its form. “Many species,” says Rymer Jones, speaking of recent sponges, “exhibiting the same porous structure, have none of the elasticity of the officinal sponge—a circumstance which is due to the difference observable in the composition of their skeletons or ramified frame-work. In such the living crust forms within its substance not only tenacious bands of animal matter, but great quantities of crystallized spicula, sometimes of a calcareous, at others of a siliceous, nature.” Thus, a frame of siliceous matter being formed by the living animal, a deposition of the same substance is continued after death.
Sea-urchins and star-fish, and numerous fossil shells, are found in these beds, which, however, differ materially from the remains of the same animals found in the earlier formations. A vast number of new species and genera of fish are also discovered in the chalk.
Nearly all the animals and plants which existed up to this period are now extinct, although they have some imperfect representatives at the present day.