890 



A TREATISE ON METAMOEPHISM. 



depletion in any of the elements of such inetamorphic sedimentary rocks, 

 this would somewhat reduce the amount of these elements and correspond- 

 ingly increase the amounts of the other elements. 



Average of analyses of '830 igneous and crystalline rocks." 



Si0 2 . 

 A1A 

 Fe 2 : 

 FeO. 

 MgO 

 CaO. 

 Na,0 



59.71 

 15.41 

 2.63 

 3.52 

 4.36 

 4.90 

 3.55 



Per cent. 



K 2 02.80 



H 2 1.52 



Ti0 2 60 



P 2 5 ' .22 



Total i 99.22 



Probably the best obtainable estimate of the composition of the muds 

 is the average of analyses of 78 Cenozoic, Mesozoic, and Paleozoic shales 

 given by Clarke. The individual shales are given weight proportional to 

 the mass of the formation, as nearly as this could be done. His results, 

 by combining the two composite analyses, one for 27 Mesozoic and Ceno- 

 zoic shales and the other for 51 Paleozoic shales, are as follows: 



Average of analyses of 78 shales. 6 ' 



a Clarke, F. W., Analyses of rocks, laboratory of the U. S. Geol. Survey, 1880-1899: Bull. U. S. 

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

 &Clarke, cit, Bull. 168, p. 17, 



