In the examination of this argument we first turned our attention to the numerous and powerful agents that are now employed in the breaking up and transporting of existing rocks. It was impossible within our narrow limits to enumerate them all. But we selected those which are at the same time the most familiar in their operations, and the most striking in their results:--mighty rivers discharging daily and hourly into the sea the accumulated spoils of vast continents; the breakers of the ocean dashing with unceasing energy against all the cliffs and coasts of the world; the tides and currents of the sea taking up the ruins which the breakers have made, and carrying them far away to the lonely depths of the ocean; the frozen rain bursting massive rocks asunder with its expansive force, and sending the fragments over lofty cliffs and steep precipices to become the prey of roaring mountain torrents, or perhaps, more fortunate, to find a place of tranquil rest on the bosom of the glittering Glacier; then this wondrous Glacier itself, a moving sea of ice, bearing along its ponderous burden from the summits of lofty mountains far down into the smiling plains, and meanwhile, with tremendous power, grinding, and furrowing, and wearing away the floor of the valley, and leaving behind it an impress which even time cannot efface; and lastly, the massive Icebergs which stud the northern and southern seas, drifting along like floating islands above the fathomless abysses of the ocean, and scattering their huge boulders over the surface of submarine mountains and valleys. All these phenomena have been learned from actual and repeated observation. They are not philosophical speculations, but ascertained facts. We cannot doubt, therefore, that the work of demolition is going on; it remains for us now to inquire about the work of reconstruction. The reader will remember that Geologists divide the stratified rocks into three distinct classes, Mechanical, Chemical, and Organic. This distinction, they say, is founded on the actual operations of Nature. From a close examination of the natural agents now at work in the world, it appears that some strata are being formed chiefly by the action of mechanical force; others chiefly by the influence of chemical laws; and others again chiefly by the intervention of organic life. Thus we have three distinct classes of rock at present coming into existence, each exhibiting its own peculiar characteristics, and each, moreover, having its counterpart among the strata that compose the Crust of the Earth. We shall now proceed to set forth some of the evidence that may be advanced in favor of these important conclusions, beginning with those rocks that are called Mechanical.