4 

 326 GEOLOGY, 



solution by rivers are extracted from the water about as rapidly as they 

 are supplied. Thus calcium carbonate is about twenty times as abun- 

 dant as sodium chloride in river-water/ but is only yj^ as abundant 

 in sea-water. 



The total river discharge into the sea is estimated at 6524 cubic 

 miles of water per year.^ This water is estimated to carry to the 

 sea annually about half a cubic mile of mineral matter in solution. At 

 this rate it would take about 9,000,000 years for the streams to bring 

 to the sea an amount of mineral matter equal to that it now contains, 

 but the proportions of the ingredients would be very different. 



The sodium chloride makes up about 2.4% of the mineral mat- 

 ter in river-water and nearly 78% of the mineral matter of the sea. 

 At this rate it would take nearly 300,000,000 years for the salt of 

 the sea to have been contributed by the rivers. It is not to be under- 

 stood, however, that this figure indicates the age of the ocean. The salt 

 is not all brought in by the rivers ; the rivers have probably not always 

 contributed at the present rate: and much salt once in the sea has been 

 precipitated. Nevertheless the above figure gives some suggestion as 

 to the order of magnitude of the figures which represent the age of the 

 ocean. 



In contrast with the salt, the amount of calcium carbonate in the 

 sea is so small that at their present rate of contribution, it would be 

 brought to the sea by rivers in about 62,000 years. 



Topography of bed. — The general relations of ocean basins to conti- 

 nents are suggested by Fig. 296. The borders of the continental plat- 

 forms are covered by the epicontinental sea, while the abysmal sea occu- 

 pies the ocean basins proper. From the figure it is seen that an ocean 

 basin is pronouncedly convex upward, and so departs as widely as may 

 be from the current notion of the homely utensil from which it is named. 

 Only when it is remembered 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 well below the mean sphere level, does the current 

 name seem justified.^ The figure also shows that the depth of an 

 ocean basin is sUght compared with the radius of the earth. 



1 Murray, Scot. Geogr. Mag., Vol. Ill, p. 76. 



2 Ibid., p. 70. 



3 Limited areas of the ocean bottom are actually concave upward; that is, they 

 are basins in the more commonly accepted sense of the term (see Chapter IX). 



