264 



THREE CRUISES OF THE " BLAKE. 



regions where the dust from deserts is blown a long distance 

 to sea. 



Where great rivers enter the ocean, the finer material is car- 

 ried out to a much longer distance than hajjpens along coasts 

 where there are but small rivers. The greensands, which owe 

 their peculiar character to the presence of glauconitic grains 

 and Q'lauconitic casts of foraminifera and other org-anisms, are 

 generally found along continental coasts where the fine silt from 

 rivers is not very abundant, as, for instance, off the Shetland 

 Islands, off the Cape of Good Hope, and off Japan, Australia, 

 and some parts of the east coast of North America. It is 

 \vithin the limited area in proximity to the continents, and oc- 

 cupied by terrigenous deposits that the varied physical condi- 

 tions are found upon which our zoogeographical divisions are 

 based. 



The deposits now forming at the bottom of the great ocean 





Fig. 180. — Pteropod ooze. 



(Murray and Renard.) 



basins far from land are made up of organic oozes or red clay. 

 The organic oozes derive their chief characteristic from the pre- 

 sence of immense numbers of dead shells, skeletons, and frus- 

 tules of pelagic organisms, which have fallen from the surface 

 waters. In the pteropod and globigerina oozes, the remains of 

 these organisms consist of carbonate of lime, which occasionally 



upon the floor of the Xorthwestern At- shore rocks become mixed with oceanic 

 lantic. Thus boulders and fragments of deposits. 



