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 happens along coasts 
where there are but small rivers. The greensands, which owe 
their peculiar character to the presence of glauconitic grains 
and glauconitic casts of foraminifera and other organisms, 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. Tt is 
within 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 zoógeographieal divisions are 
based. 
The deposits now forming at the bottom of the great ocean 
Fig. 180. — Pteropod ooze. 4, (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 Northwestern At- shore rocks become mixed with oceanic 
lantic. Thus boulders and fragments of deposits. 
