398 EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 



position is concerned, by future investigations. Quite recently a depth 

 of over 4,900 fathoms (nearly kilometers) has been recorded in what 

 is known as the Aldrich Deep, to the southeast of the Friendly Islands; 

 the greatest depth at which bottom has been reached is in 4,660 fathoms 

 (8^ kilometers) in the Atlantic, north of the Virgin Islands. A nearly 

 equal depth — 4,655 fathoms — is found in the Tuscarora Deep, to the 

 east of Japan. The Challenger's deepest sounding was in 4,475 fathoms 

 (over S kilometers), north of the Caroline Islands. On the whole I 

 estimate that about 5 per cent of the area of the ocean has a depth of 

 over 3,000 fathoms ( 5J kilometers). 



The whole surface of the earth may, from a quite general point of 

 view, be divided into elevated plateaus and submerged plains, the ele- 

 vated plateaus being represented by the continents, which occupy 

 about two-sevenths of the earth's surface, and the submerged plains by 

 the abysmal areas of the ocean, which cover about four-sevenths of the 

 earth's surface, the remaining one-seventh of the earth's surface being 

 occupied by the slope connecting the one with the other. The average 

 level of the continental plateaus is about 2,500 fathoms (over 4£ kilo- 

 meters) above the general level of the abysmal area. These great 

 troughs or hollows on the surface of the earth are filled with salt water 

 up to within a few hundred feet of the average height of the conti- 

 nental plateaus. 



At the present time the temperature of the ocean water varies from 

 28° or 29° F. (—2.22° or —1.67° C.) at the poles to from 80° to 85° F. 

 (from 26.67° to 29.44° O.) at some points within the tropics. The sea- 

 sonal variation of the surface temperature is not felt at depths over 100 

 fathoms. At the level of a depth of 1,000 fathoms beneath the surface 

 the mean temperature of the ocean is 36.5° F. (2.5° C), the Atlantic 

 and Indian Oceans being, on the whole, warmer than the Pacific at this 

 depth, and at greater depths, even within the tropics, the temperature 

 may at some points fall as low as the freezing point of fresh water. 



Sea water contains in solution a fairly constant proportion of salts 

 and a more variable proportion of gases. The saline constituents, to 

 which sea water owes its distinctive properties, consist chiefly of chlo- 

 rides and sulphates, with a comparatively small, but none the less 

 important, proportion of carbonates and bromides. Of some twenty- 

 four metals which have been detected in sea water, only sodium, mag- 

 nesium, calcium, and potassium are of any importance in determining 

 the character of the water. The presence of this saline matter gives 

 the water an increased density, and this density, measured under uni 

 form conditions, is taken as a measure of the absolute quantity of salt 

 in solution. 



In any sample of sea water the proportions of the various saline 

 constituents remain quite sensibly constant among themselves. This 

 statement, although true as a rule, is liable to an exception in the case 

 of lime, for, from actual determinations of the lime and from the increase 



