THE STRENGTH OF THE EARTH'S CRUST 



213 



tion for the porosity factor. The data, assembled from various 

 sources^ and subjected to computation, give the following results: 



TABLE VI 

 Composition or Average Igneous Rock 



Mineral Percentage 



Quartz 12.0 



Feldspars 



Orthoclase molecule 22.0 



Albite molecule 29.5 



Anorthite molecule 8.0 



Hornblende and pyroxene 16. 8 



Mica 3.8 



Accessory minerals 7.9 



TABLE VII 



Composition of Average Sediments 



* The total percentage of free silica. 



t Probably sericite in part; in that case the feldspar figure becomes lower. 



tTwo per cent clay takes 0.79 of AUOj. This requires that most of alkalies form non-aluminous 

 hydrous silicates or that 0.81 ALO3 as given by Clarke is too low. 



It is thought that the densities without porosity are figures of 

 some value for geodetic computations. The chief error in making 

 the final estimates is in connection with the lack of accurate knowl- 

 edge regarding the pore space of those sedimentary rocks not used 



I For data on the mean chemical and mineral composition of rocks see F. W. 

 Clarke, "Data of Geochemistry," Bull. 4QI, U.S. Geol. Siirv., 1911, pp. 30, 31. For 

 specific gravities of minerals see Pirsson, Rocks and Rock Minerals, 1908, p. 31; also 

 Dana, Mineralogy. For a discussion of pore space see Fuller, "Total Amount of 

 Free Water in the Earth's Crust," Water Supply Paper No. 160, U.S. Geol. Surv., 

 1906, pp. 59-72. 



