But amidst all these varieties of form and texture, the rocks we have been describing generally preserve their peculiar characteristics, and with a little experience can be easily recognized. They are found to constitute a very large part, perhaps we might say the larger part, of the stratified rocks in every country that has hitherto been explored by Geologists. Wherever we go we are met by the same familiar appearances;--beds of Conglomerate, Sandstone, Clay, Marl, Shale, recurring again and again through a series of many hundred strata, sometimes in one order, and sometimes in another; sometimes without any formation of a different kind intervening, and sometimes alternating with limestone or other rocks of which we shall speak hereafter. Such is the general character and appearance of those strata which are known among Geologists as Aqueous Rocks of Mechanical origin. Now, it must at once strike the reader, that these rocks are made up of just those very materials--the same both in kind and in form--that we have already shown to be daily prepared and fashioned by a vast and complex machinery in the great workshop of Nature. He will remember how enormous blocks are detached from the mountain side, or from the cliffs on the seashore, and broken up into fragments; how the fragments in time become pebbles, sand, and mud; and how these are caught up by rivers, tides, and currents, and carried far away to sea. Here we have certainly all the materials that are necessary for the building up of Conglomerate, Sandstone, and Shale. We have seen how they are prepared by the hand of Nature, how they are moulded into shape, how they are transported from place to place. Let us now pursue the sequel of their history, and follow them on to the end. It is plain they cannot remain forever suspended in water; sooner or later they must fall to the bottom. Yet they will not all fall together. For though all are carried downward by the one force of gravity, those materials that are smaller and lighter will be more impeded by the resistance of the water. The pebbles and coarse gravel will be the first to reach the bottom, then the sand, and last of all the fine, impalpable mud. Thus, as the current sweeps along in its course, the sediment which it bears away from the land will be in a manner sorted, and three distinct layers of different materials will be deposited in the bed of the ocean;--first, nearest to the shore, a layer of pebbles and coarse gravel, then a layer of sand, and last of all a layer of fine mud or clay. This is the first step in the construction of stratified rock. To complete the work nothing more is necessary than the consolidation of these loose and incoherent materials. If this could be accomplished, then we should have a solid stratum of Conglomerate, a solid stratum of Sandstone, and a solid stratum of Shale formed in the bed of the ocean.