290 GEOLOGY 



the amount of mineral matter which it contains may be readily 

 calculated. Assuming the average specific gravity of the mineral 

 matter to be 2.5, the 3.5% (nearly) by weight becomes about 1.4% 

 by volume, and 1.4% of 323,722,150 cubic miles is 4,532,110 cubic 

 miles. This represents the aggregate volume of mineral matter 

 in the sea 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, having been extracted from it by ground-water, and carried 

 by rivers to the sea. 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 

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

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

 in river-water, but it is only ^^ 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. 



Knowing, approximately, the amount of water discharged by 

 rivers into the sea each year (about 6,500 cubic miles *), and knowing 

 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 contrib- 

 uted by the rivers, at the present rate. It is to be understood, how- 

 ever, that this figure cannot be taken as the age of the ocean, for 

 (1) 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 <>f 

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

 of the calcium carbonate in the sea would be given to it by rivers 

 in about 62,000 years at their present rate of contribution. 



Topography of the ocean bed. The ocean basins are pro- 

 nouncedly convex upward. It is only when we remember that a 

 level surface (on the earth) is one which has the mean curvature 



1 Murray, Scot. Geog. Mag., Vol. Ill, p. 70. 



