1. Eruptive Rocks.—Crystalline, like the second, but formed at all geological periods by the irruption or intrusion of the liquid matter occupying the interior of our globe through all the pre-existing rocks.
2. Crystalline Rocks.—That portion of the terrestrial crust which was primarily liquid, owing to the heat of the globe, but which solidified at the period of its first cooling down; forming the masses known as Fundamental Gneiss, and Laurentian, &c.
3. Sedimentary Rocks.—Consisting of various mineral substances deposited by the water of the sea, such as silica, the carbonates of lime and magnesia, &c.
The mineral masses which constitute the sedimentary rocks form beds, or strata, having among themselves a constant order of superposition which indicates their relative age. The mineral structure of these beds, and the remains of the organised beings they contain, impress on them characters which enable us to distinguish each bed from that which precedes and follows it.
It does not follow, however, that all these beds are met with, regularly superimposed, over the whole surface of the globe; under such circumstances geology would be a very simple science, only requiring the use of the eyes. In consequence of the frequent eruptions of granite, porphyry, serpentine, trachyte, basalt, and lava, these beds are often broken, cut off, and replaced by others.
Denudation has been another fruitful source of change. Professor Ramsay[10] shows, in the “Memoirs of the Geological Survey,” that beds once existed above a great part of the Mendip Hills to the extent of at least 6,000 feet, which have been removed by the denuding agency of the sea; while in South Wales and the adjacent country, a series of Palaeozoic rocks, eleven thousand feet in thickness, has been removed by the action of water. In fact, every foot of the earth now forming the dry land is supposed to have been at one time under water—to have emerged, and to have been again submerged, and subjected to the destructive action of the ocean. At certain points a whole series of sedimentary deposits, and often several of them, have been removed by this cause, known by geologists as Denudation. The regular series of rock formations are, in fact, rarely found in unbroken order. It is only by combining the collected observations of the geologists of all countries, that we are enabled to arrange, according to their relative ages, the several beds composing the solid terrestrial crust as they occur in the following Table, which proceeds from the surface towards the centre, in descending order:—
ORDER OF STRATIFICATION.
| Quaternary Epoch | Modern Period. | ||
| Tertiary Epoch | – | Pliocene Period. | |
| Miocene Period. | |||
| Eocene Period. | |||
| Secondary Epoch | – | Cretaceous Rocks. | |
| Jurassic Rocks. | |||
| Triassic Rocks. | |||
| Primary Epoch | – | Permian Rocks. | |
| Carboniferous Rocks. | |||
| Devonian Rocks. | |||
| Silurian Rocks. | |||
| Metamorphic Series | – | Cambrian Rocks. | |
| Fundamental Gneiss, or Laurentian. | |||
Under these heads we propose to examine the successive transformations to which the earth has been subjected in reaching its present condition; in other words, we propose, both from an historical and descriptive point of view, to take a survey of the several epochs which can be distinguished in the gradual formation of the earth, corresponding with the formation of the great groups of rocks enumerated in the preceding table. We shall describe the living creatures which have peopled the earth at each of these epochs, and which have disappeared, from causes which we shall also endeavour to trace. We shall describe the plants belonging to each great phase in the history of the globe. At the same time, we shall not pass over entirely in silence the rocks deposited by the waters, or thrown up by eruption during these periods; we propose, also, to give a summary of the mineralogical characters and of the fossils characteristic of, or peculiar to each formation. What we propose, in short, is to give a history of the formation of the globe, and a description of the principal rocks which actually compose it; and to take also a rapid glance at the several generations of animals and plants which have succeeded and replaced each other on the earth, from the very beginning of organic life up to the time of man’s appearance.