MECHANICAL DEPOSITION.—Next let us notice what becomes of all this vast amount of clay, sand, and gravel after it is washed into the ocean. By taking up a glass of turbid water from our roadside rill, and observing that as soon as the water is undisturbed the sand and clay begin to settle, we learn that the solid matter is held in suspension by the motion of the water. But it does not remain in suspension long after being washed into the sea, for otherwise the sea would, in the course of time, become turbid for long distances from shore; and it is a well-known fact that the sea-water is usually clear and free from sensible turbidity close along shore and even near the mouths of large rivers, while at a distance of only 50 or 100 miles we find the transparency of the central ocean. Putting these facts together, we see that the ocean, nothwithstanding the ceaseless and often violent undulations of its surface, must be as a whole a vast body of still water; and to the reflecting mind the almost perfect tranquillity of the ocean is one of its most impressive features. For it is in striking contrast, in this respect, with the more mobile aerial ocean above it. We have got hold, now, of two facts of great geological importance: (1) The débris washed off the land by waves and rivers into the still water of the ocean very soon settles to the bottom; and (2) it nearly all settles on that part of the ocean-floor near the land. And now we have in view the second great office of the inorganic aqueous agencies,—deposition, the counterpart or complement of erosion. The land is the great theatre of erosion and the sea of deposition; the rocks which are constantly wasting away on the former are as constantly renewed in the latter.