400 EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 



where the salinity is over 1.027 ; but in the inclosed basins of the Medi- 

 terranean and Bed seas the salinity may reach 1.030. The average 

 salinity at the surface in the open ocean is higher than on the bottom, 

 and, like the temperature, is higher at the bottom in the Atlantic and 

 Indian Oceans than in the Pacific. 



The density of ocean water is dependent upon temperature as well 

 as pressure, and in consequence the less saline water of the polar 

 regions has a higher density than the Salter waters of the trovfics, and 

 in general the lowest annual and diurnal temperatures tend to be 

 propagated downward to the greater depths. Vertical circulation 

 downward is likewise to some extent determined by the presence of 

 detrital matter from rivers, and by the action of very constant winds 

 like those which prevail over the great Southern Ocean. 



When we examine the deposits now being laid down on the floor of 

 the ocean, we find that in all inclosed basins surrounded by continental 

 land, and for an average distance of 200 miles off continental shores 

 facing the great oceans, the deposits are for the most part made up of 

 detrital matters more or less directly derived from the subaerial denu- 

 dation of the dry land, or as a result of the destructive action of waves 

 and currents in the shallow regions of the ocean. Even when such 

 deposits are composed for the most part of carbonate of lime organisms, 

 such as shells, corallines, and corals, these bear the impress of the 

 mechanical action of the forces at work in the shallow waters, and are 

 therefore included under the general term of terrigenous deposits. The 

 predominant mineral in these terrigenous deposits is quartz, being in 

 many positions associated with glauconite; and both these minerals 

 appear to be almost wholly absent in the central parts of the great 

 ocean basins, except where the surface waters may be affected by 

 floating ice. 



The alkalies, alkaline earths, iron, manganese, and other bases, at one 

 time associated with the quartz of the sand dunes, sandstones and other 

 rocks of the continents, have been separated by chemical decomposi- 

 tion, and carried in solution or in suspension out into the abysmal 

 regions of the ocean, where they have accumulated through physical 

 or organic agencies. The lighter, less soluble, and more refractory 

 quartz has, on the other hand, accumulated on the continents and in 

 tbe deposits in their immediate vicinity. If this process has been going 

 on continuously since precipitation of water first took place on our 

 planet, the rocks on the continental areas would become more and more 

 acid in constitution and lighter, while the deposits formed at the bot- 

 tom of the ocean would become more and more basic and heavier. The 

 reason why the continents on the whole stand at a higher elevation 

 than the floor of the ocean basins may well be that, by this continu- 

 ous process, they are the lighter portions of the superficial crust, as is 

 indeed indicated by the general results of pendulum and plumb-line 

 observations. 



