I9I2.] CLARKE— SOME GEOCHEMICAL STATISTICS. 333 '•2.3C 



So far, the mechanical sediments, such as silt and sand, have 

 not been considered. From the surface of the United States, accord- 

 ing to Dole and Stabler,^'' the rivers annually carry to the sea 270,- 

 000,000 tons of dissolved substances, and 513,000,000 tons in suspen- 

 sion. If this ratio, which is only approximate, should hold for the 

 whole world, the quantity deposited in the ocean during geologic 

 time would be 102,370 X 10^- tons, and the total sedimentation, 

 chemical and mechanical, becomes 156,126 X 10^^ tons. This quan- 

 tity, distributed over the entire sedimentary area, continental and 

 oceanic, gives an average thickness of about 550 feet, or 0.000079 

 inch a year. 



The total volume of the marine sediments thus computed, is 

 13,873.000 cubic miles. The volume remaining in the ocean is very 

 nearly two thirds of this figure, 9,239,000 cubic miles. The volume 

 of all the secondary rocks derived from the decomposition of 

 igneous rocks was previously found to be 78,338,000 cubic miles. 

 Hence the portion now on the land area of the globe amounts to 

 69,099,000 cubic miles of rock, consisting in great part of materials 

 which were never transported very far from their original place of 

 formation. 



To the foregoing estimates of the oceanic sediments at least one 

 large but undetermined correction needs to be applied. The ocean 

 receives great quantities of dust, representative of aerial erosion, 

 and also quantities of volcanic ejectamenta. For these nonfluviatile 

 additions no valid estimates can yet be made. The major portion 

 of them, however, must come from disintegrated sedimentary rocks, 

 sands, and soils, and so do not affect to any serious extent our esti- 

 mates of rock decomposition. The oceanic share of the sediments 

 should be increased, but less than appears at a first glance. The 

 marine sediments now on land must include a part of the contribu- 

 tions made to the ocean by atmospheric transportation. The actual 

 distribution of the sediments is naturally very uneven. They are 

 probably thin near the margin of the red clay, and thick along the 

 continental shelves. Coral rock, for example, has been bored to a 

 depth of 1,100 feet without reaching its limit. The mechanical sedi- 

 ments are of course mainly deposited relatively near to shore. 



" U. S. Geol. Surv. Water Supply Paper, No. 234, p. 83. 



