1 68 WORK OF THE OCEAN 



would have if it were precipitated and compacted so as to have an 

 average specific gravity of 2.5. This amount of mineral matter 

 would cover the ocean bottom to a depth of about 175 feet. Its 

 amount is equal to about 20% of that of all lands above sea-level. 



A large part of the mineral matter of the sea has come from the 

 land, where it was dissolved chiefly by ground-water, and carried 

 to the sea by rivers. But the mineral matter of the sea gives no 

 more than a hint of the importance of the solvent work of water 

 in the general processes of rock decay, for most of the mineral 

 matter carried from the land to the sea in solution is taken from 

 sea-water about as rapidly as it is received. Calcium carbonate, 

 for example, is about twenty times as abundant as sodium chloride 

 in river-water, but it is only I /2oo as abundant in sea-water. This 

 is because the calcium carbonate is used by animals and plants to 

 make shells, skeletons, etc., while the salt remains in solution. 



From the amount of water discharged by rivers into the sea each 

 year (about 6,500 cubic miles), and from the amount of salt it carries, 

 it is calculated that it would take about 370,000,000 years for the 

 salt of the sea to have been contributed by rivers, at the present rate. 

 This figure, however, must not be taken as the age of the ocean, for 

 (i) the salt is not all brought in by rivers, (2) it is not probable 

 that the rivers have always contributed salt at the present rate, 

 and (3) much salt once in the sea has been precipitated. Never- 

 theless the above figure gives some suggestion as to the order of 

 magnitude of the figure which represents the age of the ocean. 



Topography of ocean basins. The ocean basins are convex 

 upward. It is only when we remember that a level surface (on the 

 earth) is one which has the mean curvature of the earth, and that 

 the deeper parts of the ocean basin are considerably below the 

 mean sphere level, that the name basin seems appropriate. 



The bed of the ocean, like the face of the land, has elevations 

 and depressions, and its deepest parts are about as far below its 

 surface as the highest mountains are above it. If the water were 

 drawn off, so that the bottoms of the ocean basins could be seen, 

 three great features would appear: (i) Extensive tracts of low land 

 (now covered by deep water); (2) other great, but less extensive 

 tracts of higher land (now covered by shallow water); and (3) 

 ridges and peaks of mountainous heights. These three principal 

 divisions may be compared to the plains, plateaus, and mountains 

 of the land, though mountain systems would be less numerous than 



