Modifications of the Surface of the Globe.

As a consequence of the hypothesis of central heat, it is admitted that our planet has been agitated by a series of local disturbances; that is to say, by ruptures of its solid crust occurring at more or less distant intervals. These partial revolutions at the surface are supposed to have been produced, as we shall have occasion to explain, by upheavals or depressions of the solid crust, resulting from the fluidity of the central parts, and by the cooling down of the external crust of the globe.

Almost all bodies, in passing from a liquid to a solid state, are diminished in size in the process. In molten metals which resume the solid state by cooling, this diminution amounts to about a tenth of their volume; but the decrease in size is not equal throughout the whole mass. Hence, as a result of the solidification of the internal parts of the globe, the outer envelope would be too large; and would no longer fit the inner sphere, which had contracted in cooling. Cracks and hollows occur under such circumstances, even in small masses, and the effect of converting such a vast body as the earth from a liquid, or rather molten condition, to a solid state, may be imagined. As the interior became solid and concrete by cooling, furrows, corrugations, and depressions in the external crust of the globe would occur, causing great inequalities in its surface; producing, in short, what are now called chains of mountains.

At other times, in lieu of furrows and irregularities, the solid crust has become ruptured, producing fissures and fractures in the outer envelope, sometimes of immense extent. The liquid substances contained in the interior of the globe, with or without the action of the gases they enclose, escape through these openings; and, accumulating on the surface, become, on cooling and consolidating, mountains of various heights.

It would also happen, and always from the same cause, namely, from the internal contraction caused by the unequal cooling of the globe, that minor fissures would be formed in the earth’s crust; incandescent liquid matter would be afterwards injected into these fissures, filling them up, and forming in the rocky crust those long narrow lines of foreign substances which we call dykes.

Finally, it would occasionally happen, that in place of molten matter, such as granite or metalliferous compounds, escaping through these fractures and fissures in the globe, actual rivers of boiling water, abundantly charged with various mineral salts (that is to say, with silicates, and with calcareous and magnesian compounds), would also escape, since the elements of water would be abundant in the incandescent mass. Added to these the chemical and mechanical action of the atmosphere, of rain, rivers, and the sea, have all a tendency to destroy the hardest rocks. The mineral salts and other foreign substances, entering into combination with those already present in the waters of the sea, and separating at a subsequent period from these waters, would be thrown down, and thus constitute extensive deposits—that is to say, sedimentary formations. These became, on consolidation, the sedimentary rocks.

The furrows, corrugations, and fractures in the terrestrial crust, which so changed the aspect of the surface, and for the time displaced the sea-basins, would be followed by periods of calm. During these periods, the débris, torn by the movement of the waters from certain points of the land, would be transported to other parts of the globe by the oceanic currents. These accumulated heterogeneous materials, when deposited at a later period, would ultimately constitute formations—that is, transported or drifted rocks.

We have ventured to explain some of the theories by which it is sought to explain the cosmography of the world. But our readers must understand that all such speculations are, of necessity, purely hypothetical.

In conformity with the preceding considerations we shall divide the mineral substances of which the earth is composed into three general groups, under the following heads:—

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.

[1] Dei corpi marini, &c., 1721.

[2] Sui crostaccei ed altri corpi marini che sè trovano sui monti, 1740.

[3] Consult Lyell’s “Principles of Geology” and the sixth edition of the “Elements,” with much new matter, for further information relative to the study of fossils during the last two centuries.

[4] “Ossements Fossiles” (4to), vol. i., p. 29.

[5] “Ossements Fossiles” (4to), vol. i., pp. 1, 2.

[6] “Ossements Fossiles,” vol. iv. (4to), p. 32.

[7] See Phil. Transactions, 1839-40-42; also, Quarterly Journal of the Geological Society, vol. viii., p. 56.

[8] “Fresh Springs of Truth.” R. Griffin and Co.

[9] Professor Tyndall in Fortnightly Review.

[10] “Memoirs of the Geological Survey of Great Britain,” vol. i., p. 297.