NO. 5 STUDY OF CHEMICAL DENUDATION — CLARKE 13 



For the average composition of the igneous rocks there are two principal 

 estimates: Clarke's/ hased upon analyses made in the laboratories of the 

 United States Geological Survey alone, and Washington's/ which includes 

 analyses from all parts of the world. These two composites, reduced to 

 100 per cent, and condensed by union of minor and unimportant con- 

 stituents, assume the following form : 



Clark^. 



Si02 59.99 



AI2O8 15.04 



Fe20s 2.59 



FeO 3.34 



MgO 3.89 



CaO 4.81 



NasO 3.41 



K2O 2.95 



H2O 1.92 



Minor constituents. . 2.06 



100.00 100.00 . 100.00 



In the mean the Survey analyses, of which many are included in Wash- 

 ington's average, evidently receive double weight. 



When the average igneous rock decomposes, it loses some substances by 

 leaching, and gains others, such as oxygen, water, and carbon dioxide. 

 The products of decomposition, as nearly as can be estimated, are 5 per 

 cent, limestones, 15 per cent, sandstones, and 80 per cent, the residues 

 which consolidate to form slates and shales. The composite analyses of 

 sedimentary rocks, made in the laboratory of the United States Geological 

 Survey,' are given in the next table, with their weighted average recal- 

 culated to 100 per cent. 



Shales. Sandstones. Limestones. Weighted mean. 



Si02 58.38 78.66 5.19 58.51 



AbOs 15.47 4.78 .81 13.07 



Fe-Os 4.03 1.08 .54 3.40 



FeO 2.46 .30 2.00 



MgO 2.45 1.17 7.90 2.52 



CaO 3.12 5.52 42.61 5.42 



Na^-O 1.31 .45 .05 1.12 



K2O 3.25 1.32 .33 2.80 



H2O 5.02 1.64 .77 4.28 



CO2 2.64 5.04 41.58 4.93 



Minor constituents 2.33 .45 .31 1.95 



100.46 100.41 100.09 100.00 



Weight 80. 15. 5. 



1 U. S. Geol. Surv., Bull. 419, pp. 4-9, 1910. ^ u. S. Geol. Siirv. Bull. 419, p. 10. 



2 U. S. Geol. Surv. Prof. Paper, No. 14, p. 106, 1903. 



