Sometimes these mysterious internal forces lifted the surface in long waves. Thus mountain chains and mountain systems were created. Often their summits, worn down by frosts and rains, disclose the core of rock which, ages before, then hot and fluid, had underlain the crust and bent it upward into mountain form. Now, cold and hard, these masses are disclosed as the granite of to-day's landscape, or as other igneous rocks of earth's interior which now cover broad surface areas, mingled with the stratified or water-made rocks which the surface only produces. But this has not always been the fate of the under-surface molten rocks, for sometimes they have burst by volcanic vents clear through the crust of earth, where, turned instantly to pumice and lava by release from pressure, they build great surface cones, cover broad plains and fill basins and valleys.
Thus were created the three great divisions of the rocks which form the three great divisions of scenery, the sediments, the granites, and the lavas.
During these changes in the levels of enormous surface areas, the frosts and water have been industriously working down the elevations of the land. Nature forever seeks a level. The snows of winter, melting at midday, sink into the rocks' minutest cracks. Expanded by the frosts, the imprisoned water pries open and chips the surface. The rains of spring and summer wash the chippings and other débris into rivulets, which carry them into mountain torrents, which rush them into rivers, which sweep them into oceans, which deposit them for the upbuilding of the bottoms. Always the level! Thousands of square miles of California were built up from ocean's bottom with sediments chiselled from the mountains of Wyoming, Colorado, and Utah, and swept seaward through the Grand Canyon.
These mills grind without rest or pause. The atmosphere gathers the moisture from the sea, the winds roll it in clouds to the land, the mountains catch and chill the clouds, and the resulting rains hurry back to the sea in rivers bearing heavy freights of soil. Spring, summer, autumn, winter, day and night, the mills of Nature labor unceasingly to produce her level. If ever this earth is really finished to Nature's liking, it will be as round and polished as a billiard ball.
From a photograph by Bailey Willis
Years mean nothing in the computation of the prehistoric past. Who can conceive a thousand centuries, to say nothing of a million years? Yet either is inconsiderable against the total lapse of time even from the Archean's close till now.
And so geologists have devised an easier method of count, measured not by units of time, but by what each phase of progress has accomplished. This measure is set forth in the accompanying table, together with a conjecture concerning the lapse of time in terms of years.