THE SEA BOTTOM 57 



matter having been dissolved away on its passage downward 

 through the water. In certain areas, notably in the tropical 

 regions of the Pacific and Indian oceans, there are great num- 

 bers of minute animals known as Radiolarians in the surface 

 waters ; these have skeletons of silica, often of very beauti- 

 ful lattice design, which form the most conspicuous part 

 of the deep-sea ooze in these regions (Plate 21). Finally, 

 at the greatest depth of all (below 2,700 fathoms) even the 

 silica is dissolved and the deposits then consist exclusively 

 of what Murray called " red clay." This is a true clay 

 which has been formed by the prolonged action of the sea 

 water on volcanic dust which is all that has survived the 

 long journey from the surface to the bottom. Like the 

 siliceous deposits it is present at all depths but is usually 

 masked by the deposits of animal or plant origin. In this 

 red clay are found spherules probably of meteoritic origin, 

 and also the completely insoluble teeth of sharks — some of 

 which belong to extinct species — and ear-bones of whales. 

 Although the actual substance of the sea bottom has not 

 the same overwhelming importance as the soil on land, since 

 even the sea weeds do not root in it, merely attaching 

 themselves to rocks, and obtain all their nourishment from 

 the substances in solution in the water, yet it has consider- 

 able importance. In the first place it is of use to the bottom 

 living animals which may crawl on its surface, burrow into 

 it. or attach themselves to it. The nature of the bottom- 

 sand, mud, gravel or rock — is of the first importance to 

 them. Most ot the animals collected from the bottom do 

 not live on the surface of this but burrow into it. Each bur- 

 rower is adapted for excavating in a particular substratum 

 — sand, shell gravel, muddy shell gravel, or mud. This ex- 

 plains why we invariably collect a particular group of 

 animals from each of these localities. In some cases an ap- 

 parently trifling difference between two species may enable 



