178 THE WORLD OF LIFE CHAP. 



up from all depths, and especially along lines at right angles 

 to the shore at short distances from each other. The exact 

 physical and chemical nature of all these samples was 

 accurately determined, and some most curious results were 

 brought to light. 



The earlier geologists had assumed, in the absence of 

 direct evidence to the contrary, that the suspended matter 

 poured into the sea by rivers was, sooner or later, by means 

 of winds and waves and ocean currents, distributed over the 

 whole of the ocean floors, and was gradually filling up or 

 shallowing the oceans themselves. But the Challenger re- 

 searches showed that this idea was almost as remote as 

 possible from the truth. The actual facts are, that the whole 

 of the land debris, with a few special and very minute ex- 

 ceptions, are being deposited on the sea-bottom very near 

 the shore, comparatively speaking, and all but the very finest 

 material quite close to it. Everything in the nature of 

 gravel or sand, of which so much of the rocky strata consists, 

 is laid down within a very few miles, only the finer muddy 

 sediments being carried so far as from 20 to 50 miles from 

 land ; while the very finest of all, under the most favourable 

 conditions, rarely extends beyond 150 and never exceeds 

 300 miles from land into the deep ocean. Mr. A. Agassiz 

 also, has found that the extremely fine mud of the Mississippi 

 River is never carried to a greater distance than 100 miles 

 from its mouth. If we take even so much as 50 miles for 

 the average distance to which the denuded matter is carried, 

 we find the whole area of deposit around South America to 

 be 60,000 square miles. But the area of that continent is 

 about six million square miles, so that deposition goes on 

 about a hundred times as fast as denudation ; while over 

 considerable areas where the deposits are of a sandy rather 

 than of a muddy or slaty nature, it may go on a thousand 

 times as fast. This is a most important fact which does not 

 appear to have been taken into full consideration by geologists 

 even to-day. 



The correlative fact as to the ocean bed is, that over the 

 whole of it, when more than the above-named distances from 

 land, what are called " deep-sea oozes " are found. These 

 are formed almost entirely by the calcareous or silicious 



