1906] 



IN CHEMICAL GEOLOGY. 23 



embarrassing. Its magnitude is easily estimated, and it gives a 

 measure of the extent to which the igneous rocks have been de- 

 composed. 



If we assume that all of the sodium in the ocean was derived 

 from the leaching of the primitive rocks, and that the average com- 

 position of the latter is correct, it is easy to show that the marine 

 portion is very nearly one thirtieth of that contained in the ten-mile 

 lithosphere. That is, the complete decomposition of a shell of igne- 

 ous rock, one third of a mile thick, would yield all the sodium in 

 the ocean. Some sodium, however, is retained by the sediments, 

 and the analyses show that it is about one third of the total amount. 

 That is, the oceanic sodium represents two thirds of the decomposi- 

 tion, and the estimate must, therefore, be increased one half. On 

 this basis, a rocky shell one half mile thick, completely enveloping 

 the globe, would slightly exceed the amount needed to furnish the 

 sodium of the sea and the sediments. No probable change in the 

 composition of the lithosphere can modify this estimate very con- 

 siderably ; and since the ocean may contain primitive sodium, not 

 derived from the rocks, the half mile must be regarded as a maxi- 

 mum allowance. If the primeval rocks were richer in sodium than 

 those of the present day, a smaller mass of them would suffice ; if 

 poorer; more would be needed to account for the salt in the sea. 

 Of the two suppositions, the former is the more probable ; but 

 neither assumption is necessary. If, however, we assume that our 

 igneous rocks are not altogether primary, but that some of them 

 represent re-fused or metamorphosed sedimentaries, we must con- 

 clude that they have been partly leached and have, therefore lost 

 sodium. That is, the original matter was richer in sodium, and the 

 half-mile estimate is consequently too large. 



From another point of view, the thinness of the sediments can 

 be simply illustrated. The superficial area of the earth is 199,712,- 

 000 square miles, of which 55,000,000 are land. According to 

 Geikie,^ the mean elevation of all the continents is 2,411 feet. 

 Hence, if all of the land now above sea level were spread uniformly 

 over the globe, it would form a shell about 660 feet thick. If we 



^"Textbook of Geology," 4th ed., vol. i, p. 49. 



