You may readily enough understand how it is that soundings are made, though you can see how difficult it must be to work a sounding line several miles long. Yet men are able not only to measure the depth of the water, but by means of the instrument called a dredge, to bring up bucketfuls of whatever may be lying on the sea floor, from even the deepest parts of the ocean. In this way during the last few years a great deal of additional knowledge has been gathered as to the nature of the sea floor, and the kind of plants and animals which live there. We now know that even in some of the deepest places which have yet been dredged there is plenty of animal life, such as shells, corals, star-fishes, and still more humble creatures. We can not, indeed, examine the sea bottom with anything like the same minuteness as the surface of the land. Yet a great deal may be learnt regarding it. If you put together some of the facts with which we have been dealing in the foregoing lessons, you may for yourselves make out some of the most important changes which are in progress on the floor of the sea. For example, try to think what must become of all the wasted rock which is every year removed from the surface of the land. It is carried into the sea by streams, as you have now learnt. But what happens to it when it gets there? From the time when it was loosened from the sides of the mountains, hills, or valleys, this decomposed material has been seeking, like water, to reach a lower level. On reaching the hollows of the sea bottom it can not descend any further, but must necessarily accumulate there. It is evident, then, that between the floor of the sea and the surface of the land, there must be this great difference: that whereas the land is undergoing a continual destruction of its surface, from mountain crest to sea shore, the sea bottom, on the other hand, is constantly receiving fresh materials on its surface. The one is increased in proportion as the other is diminished. So that even without knowing anything regarding what men have found out by means of deep soundings, you could confidently assert that every year there must be vast quantities of gravel, sand and mud laid down upon the floor of the sea, because you know that these materials are worn away from the land. Again, you have learnt that the restless agitation of the sea is due to movements of the air, and that the destruction which the sea can effect on the land is due chiefly to the action of the waves caused by wind. But this action must be merely a surface one. The influence of the waves can not reach to the bottom of the deep sea. Consequently that bottom lies beyond the reach of the various kinds of destruction which so alter the face of the land. The materials which are derived from the waste of the land can lie on the sea floor without further disturbance than they may suffer from the quiet flow of such ocean currents as touch the bottom.