The deposits on the deep sea bottom are very largely either organic or the shells and skeletons of organisms which have fallen to the bottom from near the surface as already explained. Most common of these are the deep sea "oozes" which are made up of the remains and shells of tiny organisms called "foraminifers." These "oozes" cover about 50 million square miles of the sea bottom down to depths of from two to three miles. At depths greater than from two to three miles, a peculiar red clay is the prevailing deposit. This is most extensive of all, covering an area of 55 million square miles, or nearly the total area of lands of the earth. Some remains of organisms are mixed with this clay, but since most of the shells are of carbonate of lime and very thin, they are dissolved without reaching the bottom in the deep sea water which is under great pressure and rich in carbonic acid gas. The deep sea deposits, both "oozes" and red clay, do, however, contain some land-derived and other materials. Thus off the west coast of Africa some dust carried by the prevailing winds from the Sahara Desert, is known to fall in the deep sea several hundred miles from shore. Volcanic dust is carried for many miles and deposited in the deep sea--particularly in the South Pacific Ocean. Bits of porous volcanic rock called "pumice" sometimes float long distances out over the deep sea, before becoming water soaked. Icebergs often drift far out from the polar regions over the deep sea, and on melting the rock débris which they carry is dropped to the sea bottom. Also, particles of iron and dust from meteorites ("shooting stars") have been dredged from the deep sea. One important geological significance of the deep sea deposits is the proof which they furnish that, from at least as far back as the beginning of the Paleozoic era, fully twenty-five million years ago, to the present time, the two great deep ocean basins--the Atlantic and the Pacific--have maintained essentially the same positions on the earth. This is proved by the fact that nowhere, on any continent among the rocks of all ages, as old at least as the early Paleozoic, do we find any really typical deep-sea deposits. There is then no evidence that a deep sea ever spread over any considerable part of any continent, and this in spite of the fact that marine deposits of shallow water origin furnish abundant evidence of former sea extensions. The shallow seas have at various times spread over large portions of the continents.