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. The uppermost group, which has been called the supercretaceous or tertiary formation, appears in our island to have been formed during four great eras, as we find fresh-water deposits alternating with marine ones. The term _eocene_, which is the first or oldest deposit; _miocene_, which is the second; _pliocene_, which is the third; and the _newer pliocene_,--which is the fourth and last, have been applied to these formations, the names referring to the respective proportions of existing species found among their fossil shells.[242]