66 GEOLOGICAL DISTRLBUTION, 



CHAPTER VIIl. 



GEOLOGICAL DISTRIBUTION. 



'"T^HE past history of animals might conceivably have 

 J^ been a sealed book to man's investigations but 

 fortunately the succession of organisms has left considerable 

 vestiges behind it. These vestiges, in a general way, are 

 termed fossils, which are mostly found deposited in earth. 

 The surface of the earth for a slight but varying depth 

 consists of a loose soil, but below this there are layers or 

 strata, formed of various substances, such as limestone, 

 sandstone, coal and so on. These strata have been gradually 

 deposited in past ages by the action of natural forces. At 

 the present time the same process is going on. The dry 

 land is slowly being broken up by the action of rain, frost 

 and other agencies, and the finely divided remains are being 

 carried out to the sea by rivers. There the sediment in the 

 form of mud and sand is slowly deposited on the sea-floor. 

 All along the sea coasts the waves are ceaselessly carrying 

 on the same work of destruction, the pebbles, sand and 

 mud being deposited out to sea. Hence the physical 

 agencies of wind, tide, rain and wave work to a common 

 end — the reduction of the earth's surface to a dead level 

 which, if ever attained, will be some feet below the general 

 surface of the sea. At present there is a counteracting 

 force to the attainment of this in the elevation of the earth's 

 surface by the active agencies in its interior. 



We must therefore conceive of the whole of the earth's 

 surface as a shifting scene of land and water, upon which 

 the levelling and elevating agencies are constantly at work 

 in opposite directions. Should the elevating agencies, due 



