192 THE WORLD OF LIFE 



nuded matter that lowers the continental snrfaces at so rapid 

 a rate, and is poured into the sea at various points around 

 their coasts ; and this is the more necessary because recent re- 

 searches on this matter have led to results as surprising as those 

 of the measurement of the amount of denudation bv rivers. 



During the voyage of the Challenger round the world for 

 the purpose of oceanic exploration, not only was the depth of 

 the great oceans determined by numerous lines of soundings 

 across them in various directions, but, by means of ingenious 

 apparatus, samples of the sea-bottom w^ere brought 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 researches showed 

 that this idea was almost as remote as possible from the truth. 

 The actual facts are, that the wdiole of the land debris, with a 

 few special and very minute exceptions, are being deposited on 

 the sea-bottom very near the shore, comparatively speaking, 

 and all but the very finest material quite close to it. Every- 

 thing 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; w^hile 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, w^e find the whole area of deposit around South 



