xii GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 343 



when first discovered, this fact constituting the test of the 

 class to which an island belongs ; whence he argued that none 

 of them had ever been connected with continents, but all had 

 originated in mid-ocean. These considerations alone render 

 it almost certain that the areas now occupied by the great 

 oceans have never, during known geological time, been 

 occupied by continents, since it is in the highest degree im- 

 probable that every fragment of those continents should have 

 completely disappeared, and have been replaced by volcanic 

 islands rising out of profound oceanic abysses ; but recent 

 research into the depth of the oceans and the nature of the 

 deposits now forming on their floors, adds greatly to the 

 evidence in this direction, and renders it almost a certainty 

 that they represent very ancient if not primaeval features of 

 the earth's surface. A very brief outline of the nature of this 

 evidence will be now given. 



The researches of the Challenger expedition into the 

 nature of the sea -bottom show, that the whole of the land 

 debris brought down by rivers to the ocean (with the ex- 

 ception of pumice and other floating matter), is deposited 

 comparatively near to the shores, and that the fineness of the 

 material is an indication of the distance to which it has been 

 carried. Everything in the nature of gravel and sand is laid 

 down within a very few miles of land, only the finer muddy 

 sediments being carried out for 20 or 50 miles, and 

 the very finest of all, under the most favourable conditions, 

 rarely extending beyond 150, or at the utmost, 300 miles 

 from land into the deep ocean. 1 Beyond these distances, and 

 covering the entire ocean floor, are various oozes formed wholly 

 from the debris of marine organisms ; while intermingled with 

 these are found various volcanic products which have been 

 either carried through the air or floated on the surface, and a 

 small but perfectly recognisable quantity of meteoric matter. 

 Ice-borne rocks are also found abundantly scattered over the 

 ocean bottom within a definite distance of the arctic and 

 antarctic circles, clearly marking out the limit of floating ice- 

 bergs in recent geological times. 



1 Even the extremely fine Mississippi mnd is nowhere found beyond a 

 hundred miles from the mouths of the river in the Gulf of Mexico (A. Agassiz, 

 Three Cruises of the Blake, vol. i. p. 128). 



