In the world then, as it does now, water acting on the dry land produced remarkable changes. We have evidence of extensive districts over which the most luxuriant vegetation must have spread for ages,--from the remains of plants in every state of decay,--which we find went to form our great coal-fields. These, by some changes in the relative levels of land and water, became covered with this fluid; and over this mass of decaying organic matter, sand and mud were for ages being deposited. At length, rising above the surface, it becomes covered with vegetation, which is, after a period, submerged; the same deposition of sand and mud again takes place, it is once more fitted for vegetable growth, and thus, cycle after cycle, we see the dry land and the water changing places with each other. This will be evident to every one who will carefully contemplate a section of one of the coal-fields of Great Britain. We find a stratum of coal lying upon a bed of under clay, and above it an extensive stratum of shale or sandstone, probably formed by the denudation of the neighbouring hills; and in this manner we have many strata of coal, shale, clay, ironstone, and sandstone alternating with each other; the coal-formations of the South Wales coal-field having the extraordinary thickness of 1500 feet. The lowest bed of this extensive series must at one time have been exposed as the surface of the country. Ascending in the series, we have now formations of a more recent character, in which fishes of a higher order of organization, creeping and flying saurians, crocodiles and lizards, tortoises, serpents, and frogs, are found. The lias formations (a term corrupted from _layers_), consisting of strata in which an argillaceous character prevails, stand next in series. In these we have animals preserved in a fossil state, of a distinguishingly different character from those of the inferior strata. We meet with extended beds of pentacrinites, some inches in thickness; and their remains are often so very complete that every part of the skeleton can be made out, although so complicated that it cannot consist of less than 150,000 parts. In these formations we often find the curiously beautiful remains of the ammonites, of which a great variety have been discovered. Of the belemnites--animals furnished with the shell and the ink-bag of the cuttle-fish, with which it darkened the water to hide itself from enemies, numerous varieties have also been disentombed, with the ink-bag so well preserved, that the story of the remarkable fossil has been written with its own ink. In addition to these we find nautili; and sixty species of extinct fishes have been described by Agassiz from the lias of Lyme Regis alone.