222 



CLARKE— SOME GEOCHEMICAL STATISTICS. [April 20, 



be considered at some future time when suitable data are at hand. 

 For present purposes they are negUgible. 



■ The mean composition of the sedimentary rocks has been deter- 

 mined in a dilTerent way. Composites of many samples were pre- 

 pared and analyzed as if they were single samples, a method which 

 saved much labor and was as satisfactory as if numerous individual 

 analyses had been made. The data thus obtained have been repeat- 

 edly published, except in the case of the shales, for which a new 

 estimate is here given. The analyses, recalculated to 100 per cent., 

 are as follows : 



(A) Composite of 78 shales plus 45 individual analyses taken from 



the laboratory records of the U. S. Geological Survey. 

 123 shales in all. 



(B) Composite analysis of 253 sandstones. 



(C) Composite analysis of 345 limestones. 



Si02 



AI2O3 



FesOa 



FeO 



MgO 



CaO 



Na20 



K2O 



H2O 



Ti02 



CO2 



P2O5 



MnO 



Minor constituents 



59-23 

 15.68 



3-59 

 2.85 

 2.71 

 2.58 

 1.27 

 3-14 

 4-97 

 •65 

 2.40 



•15 

 ■ 05 

 • 73 



78.33 

 4.76 

 1.07 



.30 

 1. 16 

 5.49 



.45 

 1.31 

 1.66 



.25 



5.02 



.08 



.12 



5-18 

 .81 



}.S4 



7.90 



42.57 



•05 



•33 



.77 

 .06 



41.54 

 .04 



•05 

 .16 



100.00 



100.00 



100.00 



These three columns represent the three dominant types of sedi- 

 mentary rocks, and differ from the figures given the igneous rocks 

 in certain losses by leaching, mainly of lime, magnesia and alkalies, 

 and in a gain of water and carbon dioxide received from percolating 

 waters and the atmosphere. There is a small increase in volume 

 consequent upon the gains ; the materials lost are principally to be 

 sought for in that universal reservoir, the ocean. 



For the relative proportions of the sedimentary rocks several 



