314 CONDITION OF THE FLOOR OF THE OCEAN. 



is higher than 35° F. In depths be^^ond 2,000 fathoms the average 

 temperature over the tioor of the North Atlantic is about 2'^ F. above 

 the average temperature at the bottom of the Indian Ocean and 

 South Atlantic, while the average temperature of the bed of the 

 Pacific is intermediate between these. 



It is admitted that the low temperature of the deep sea has been 

 acquired at the surface in polar and subpolar regions, chiefly within 

 the higher latitudes of the Southern Hemisphere, where the cooled sur- 

 face water sinks to the bottom and spreads slowly over the floor of 

 the ocean into equatorial regions. These cold waters carry with them 

 into the deep sea the gases of the atmosphere which are everywhere 

 taken up at the surface, according to the known laws of gas absorption. 

 In this way myriads of living animals are enabled to carry on their 

 existence at all depths in the open ocean. The nitrogen remains more 

 or less constant at all times and places, but the proportion of oxygen 

 is frequently much reduced in deep water, owing to the processes of 

 oxidation and respiration which are there going on. 



The deep sea is a region of darkness as well as of low temperature, 

 for the direct rays of the sun are wholly absorbed in passing through 

 the superficial layers of water. Plant life is in consequence quite 

 absent over 93 per cent of the bottom of the ocean, or 66 per cent of 

 the whole surface of the lithosphere. The abundant deep-sea fauna, 

 which covers the floor of the ocean, is, therefore, ultimately dependent 

 for food upon organic matter assimilated by plants near its surface, in 

 the shallower waters near the coast lines, and on the surface of the 

 dry land itself. 



As has been already stated, about 7,000,000 square geographical 

 miles of the sea floor lies within the 100-fathom line, and this area is 

 in consequence subject to seasonal variations of temperature, to strong 

 currents, to the effects of sunlight, and presents a great variety of 

 physical conditions. The planktonic plant life is here reinforced by 

 the littoral seaweeds, and animal life is very abundant. About 40 per 

 cent of the water over the bottom of this shallow-water area has a 

 mean temperature under 40° F. , while 20 per cent has a mean temper- 

 ature between 40° and 60° F., and 40 per cent a temperature of over 

 60° F. 



It follows from this that only 3 per cent of the floor of the ocean 

 presents conditions of temperature favorable for the vigorous growth 

 of corals and those other benthonic organisms which make up coral 

 reefs and require a temperature of over 60° F. all year round. On the 

 other hand, more than half of the surface of the ocean has a tempera- 

 ture which never falls below 60° F. at any time of the year. In these 

 surface waters, with a high temperature, the shells of pelagic mol- 

 lusks, f oraminif era, algjfi, and other planktonic organisms are secreted 

 in great abundance and fall to the bottom after death. 



