COMPOSITION OF THE LITHOSPHERE. 



937 



Amounts of the eleven most important oxides of the lithosphere, as estimated in 1891 



and 1900. a 



* Including hygroscopic water; probably 0.40 per cent. 



Clarke further made a rough approximation of the proportion of the 

 more important minerals in 500 igneous rocks, as follows: 6 



Important minerals in BOO igneous rocks. 



Per cent. 



Quartz 12. 



Feldspars ■ 60. 



Pyroxenes and amphiboles 18. 



Micas 4.0 



. Total ..• ! 94.0 



The foregoing' estimates are wholly based upon the analyses of "erup- 

 tive and crystalline"- rocks, and represent, according to Clarke's point of 

 view, the average composition of the "primitive crust of the earth." In these 

 analyses all of the unmetamorphosed and metamorphosed sediments, so far 

 as they are recognized as such, are excluded. As we shall see later, the 

 unmetamorphosed sediments probably compose as much as one-thirtieth 

 and the unmetamorphosed and metamorphosed sediments may together com- 

 prise as much as one-fifteenth of the lithosphere. The average composition 

 of the sediments is of the utmost importance in reference to a redistribution 

 of the elements. Clarke has had made six composite analyses of groups of 



"Clarke, F. W., Analyses of rocks, laboratory of the U. S. Geol. Survey, 

 Geol. Survey No. 168, 1900, p. 14. 

 t> Clarke, cit., p. 16. 



-1899: Bull. U. S. 



