It is often long before the stuff brought down by the rivers settles on the floor of the ocean. The finer particles may be carried out to sea for three hundred miles or more before they settle. Within this three-hundred-mile zone the land-derived materials are distributed over the floor in orderly succession. Nearer the land we shall find coarse gravel and sand. Beyond there will be tracts of finer sand and silt, with patches of gravel here and there. Still farther off will come fine blue and green muds, which are made of the tiny particles of such materials as form the ordinary rocks of the land. But when we are once past this zone of land material we come upon deposits which are the ocean's own freehold--materials which it does not derive from the continents, but which may be called oceanic in origin. First there are vast sheets of exceedingly fine red and brown clay. Whence comes it? It is by far the most common deposit in all the deeper parts of the ocean. It may either be the dust of volcanic fragments washed away from volcanic islands, or (which is much more likely) it may be supplied by eruptions under the sea. For it must be remembered that the sea floor is two to five miles nearer the hot rocks that are in the interior of the earth than the land surface is, and that consequently the water coming into contact with them may cause explosions arising from the action of steam. This is a question we shall have to consider later, and for the present we must ask the reader to accept the fact--and read on. There is one very curious thing about this red clay, and it is that the accumulations of it appear to be built up very slowly. Where it occurs farthest from land great numbers of sharks' teeth with ear bones and other bones of whales have been dredged up from it. Some of these relics are quite fresh; others are coated with a crust of brown peroxide of manganese. Some are covered with this material and hidden in it. One haul of an ocean dredge will bring up the bones in all these states, so that they must be lying side by side. The bones are probably those of many generations of animals, and it must take a long time to cover them with the manganese deposit. But the clay is deposited even more slowly than the manganese, so that it must fall very slowly indeed.