Assuming, then, that the earth was in a state of incandescence when it began to take a definite form, we shall at once see that the denser materials composing it would gravitate towards the centre, forming a semi-plastic mass surrounded by an envelope of gases and watery vapour. The gases would be quickly disposed of in various chemical combinations, and the watery vapour would be condensed and deposited in depressions on the surface of the central mass as soon as it had become cooled sufficiently. The outer crust of this central, semi-solid mass was soon converted, under the intense heat, into a hard, granite-like rock, which was continually subject to sudden upheavals, resulting partly from the violent escape of gases, and partly from water passing through fissures on the surface to the heated interior and giving rise to steam of great expansive power. In this manner great inequalities of the surface were, no doubt, produced, whose rugged edges, after the lapse of a vast period of time, were gradually softened down by the subsequent action upon them of air and water. This first rock formation is termed by geologists the Plutonic (from Pluto, monarch of hell), on account of its being the result of intense heat, and not, as is the case with all other rock formations, laid down in layers by water. Whether the Plutonic rock forms a solid centre to our earth is matter of uncertainty; but all are agreed that the internal heat of our planet, whether caused by the friction of the particles of a solid substance or by a molten fluid, is still, even in these later times, intense. In boring through the earth’s crust, the average increase in temperature for every fifty feet of descent, after the first hundred feet from the surface, is one degree Fahr., which would give us, at a depth of 125 miles, sufficient heat to melt most of the rocks. This intense internal heat has generated, in times long gone by, enormous forces, by which rocks of all ages have been raised and depressed, twisted and distorted, broken and forced out of position, and forcibly compressed, so as to eventually cause most important changes of surface level.
The next class of rock-formation is totally different from the Plutonic, or unstratified series, in that it is the result of the wear and tear of the surface when acted upon by air and water, and is laid down, in the first instance, by water, as sediment. Water, in the forms of seas, rivers, rain, and ice, has been the chief agent in the arrangement of all the stratified rocks, the determination of the earth’s contour, the direction of valleys, and, in fact, the regulation of the whole physical geography of the visible portion of the earth. With the help of this mighty agent, so soon as the earth had become sufficiently cool to permit condensation to take place in its vapoury envelope, the ceaseless wear and tear of the Plutonic—and, subsequently, of all other—rocks, which has accumulated so vast a mass of material, commenced. Large volumes of water were gradually deposited, without intermission, until permanent seas and rivers had become established, and the new process of stratification, which was henceforth destined to shape the crust of the earth and to provide the conditions of life, commenced to operate. This action is taking place daily in rivers and seas, as we may observe at any time. On the tops of mountains the same action is in operation, though under different conditions, snow and ice splitting fragments from the rocks to be borne away as grit into the valleys by impetuous torrents and deposited in other places. Within the Polar circles ice on a grander scale is levelling down the land; glaciers, covering thousands of square miles, are slowly sliding down the valleys, grinding their surfaces still deeper—forming sands, clays, and gravels, and forcing these down to the sea-shore; and icebergs, many miles in circumference, are carried by currents along coasts and against cliffs like huge ploughs, completely altering the face of the rocks beneath. This wear and tear results in the formation of immense quantities of detritus, which is deposited in layers at the bottom of seas and rivers, and consolidated by pressure, being frequently assisted by lime, iron, or silica as a cement. The coarser-textured rock has been laid down in rapidly-moving, shallow water; and the finer-textured in still, deep water. Thus, through many long ages—probably millions of years—the surface of the earth underwent continual change from the constant deposition of stratified rock, each layer of which completely buried beneath it the various life forms of the previous period, which circumstance enables us to ascribe to the various members of the animal and vegetable kingdoms particular geological periods; for fossilised remains of animals and vegetables have been unearthed in the different layers of the stratified rocks, conclusively proving their existence on the earth at those periods.
In the Plutonic or unstratified rock-formation period there was, of course, no life upon the earth, the conditions necessary for such development not being present; but in the very earliest of the stratified formations we find evidence of the dawn of marine life, both vegetable and animal. Geologists have divided the stratified rock into three chief divisions, the Palæozoic (ancient life), or Primary; the Mesozoic (middle life), or Secondary; and the Kainozoic (latest life), or Tertiary. Each of these, again, has been subdivided into smaller sections, according to the particular kind of deposit met with, the particular places where the best examples are to be found, or the particular life-forms existing. The Primary, the depth of which is unknown, is subdivided into seven periods—viz.:—
Laurentian, consisting of highly metamorphosed (that is, changed in appearance from the original stratified rock character, owing to its proximity to the molten Plutonic rock) limestone, containing fossil remains of the Foraminifera, some of the first living organisms.
Huronian, consisting of less highly metamorphosed sandstone, limestone, etc., and containing fossil remains of lowly-organised molluscs (soft-bodied organisms).
Cambrian, consisting of slates, sandstones, and conglomerates, and containing fossil remains of sponges, sea-weeds, star-fishes, sea-lilies, lowly shell-fish, marine worms, and the first land plants.
Silurian, consisting of slates, limestones, etc., and containing fossil remains of corals, chambered spiral shell-fish, crabs, sea-worms, and bony plates and scales of a low form of fish.
Devonian, consisting of old red sandstone, shales, and coralline limestone, and containing fossil land plants, fishes, belonging to shark, ray, and sturgeon families, and first fossil insect.
Carboniferous, consisting of mountain limestone, coal, sandstone, ironstone, clays, etc., and containing fossil scorpions, beetles, and amphibians.
Permian, consisting of new red sandstone, marls, magnesian limestones, etc., and containing fossils of true reptiles.