I9I2.] 



CLARKE— SOME GEOCHEMICAL STATISTICS. 



223 



estimates have been made, which, however, can only be regarded as 

 rough approximations to the truth. By studying the way in which 

 an average igneous rock can break down, I have computed that the 

 shales form 80 per cent., the sandstones 15 per cent., and the lime- 

 stones 5 per cent, of the total.*' Van Hise'' distributes them as 65, 

 30 and 5 per cent, respectively, and Mead,^ by a graphic process, has 

 found 80, II and 9 per cent. Combining the composite analyses just 

 given in accordance with these ratios the following mean values are 

 obtained for the total composition of the sedimentaries. The fourth 

 column contains the average of the three separate estimates. 



Clarke. 



Van Hise. 



Mead. 



Average. 



SiOz 



AI2O3 



FezOs 



FeO 



MgO 



CaO 



NajO 



K2O 



H2O 



Ti02 



CO2 



P2O5 



MnO 



Minor constituents 



59-39 

 13-30 

 3.06 

 2-33 

 2-74 

 5-01 

 1.09 



2.73 

 4.27 

 -56 

 4-75 

 •13 

 •05 

 •59 



62.29 



11.67 



2.69 



1.94 



2.50 



5-46 



.96 



2-45 



3-77 



.42 



5-15 

 .12 



•05 

 •53 



56.47 

 13-14 

 3-04 

 2.31 

 3.00 

 6.50 

 1.07 

 2.69 

 4^23 

 •55 

 6.21 



• 13 

 •05 

 .61 



5938 

 12.70 



2-93 

 2.19 



2-75 



5-65 



1.04 



2.63 



4.09 



•SI 



5-37 



• 13 



•05 



•58 



100.00 



100.00 



100.00 



100.00 



The first of these averages is the most probable, since it harmon- 

 izes best with the mean composition of the igneous rocks. If we 

 reduce it to the form adopted for the latter, by rejecting its acces- 

 sions, carbon dioxide and water, and also neglecting the minor con- 

 stituents, it may be compared with the average marked E as follows : 



Igneous E. Sedimentaries. 



Si02 61.68 65.70 



AI2O3 15-41 14.72 



Fe203 2.67 3.38 



FeO 3.55 2.58 



MgO 3-97 3-03 



CaO 5.02 5.54 



'U. S. Geol. Surv., Bull. 491, p. 31. 



' U. S. Geol. Surv., Monograph 47, p. 940. 



^ Journ. Geol., 15, 238. 



