But before giving any examples of these abnormal deposits we must first study the modern normal deposits; before we can rightly understand the sharp contrast between the ancient and the modern action of the elements, we must become familiar with the way in which fossils are now being buried by our rivers and oceans. One of the many geological myths dissipated by the work of the "Challenger" Expedition, which, as Zittel says, "marks the grandest scientific event of the nineteenth century," is that about the ocean bottom and the work now being carried on there. The older text-books taught that, not only was the bottom of the ocean thickly strewn with the remains of the animals which died there and in the waters above, but also that the oceanic currents were constantly wearing away in some places and building up in others over all the ocean floor, and hence producing true stratified deposits. Accordingly it was said that it was only necessary for these beds to be lifted above the surface to produce the ordinary rocks that we find everywhere about us. But we now know that the ocean currents have, as Dana says, "no sensible, mechanical effects, either in the way of transportation or abrasion."[55] We know also that all kinds of sediment drop so much quicker in salt water than in fresh, that none of it gets beyond the narrow "continental shelf" and the classic 100 fathom line, which in most cases is not very far from shore. In the north Atlantic there are sediments found in deeper water produced by ice-floes or icebergs dropping their loads there; but we cannot suppose such work to have gone on when the Arctic regions were clothed with a temperate-climate vegetation, much less that such things occurred over all the earth. On the floor of the open ocean, and away from the tracks of our modern icebergs, we have two or three kinds of mud or ooze formed from minute particles of organic matter; but besides these =absolutely nothing= save a possible sprinkling of volcanic products, which of course are limited in their distribution. Where then can we find a stratified or bedded structure now being formed over the ocean bottom? Dana says there is nothing of the kind now being produced there, save as the result of possible variations during the passing ages in the organic deposits thrown down, where a bed of ooze may be supposed to be thrown down directly upon another kind of ooze. There is =no gravel=, =no sand=, =no clay=, but whatever variation there might be in the organic deposits, the new kind would be laid down immediately upon the preceding similar deposits, unless a thin sprinkling of volcanic dust happened to intervene.