I906J 



IN CHEMICAL GEOLOGY. 17 



significance as a part of all terrestrial matter, are maxima ; and tend 

 therefore to compensate for the omission of factors which could not 

 well be estimated directly. 



The average composition of the lithosphere is very nearly that 

 of the igneous rocks alone. The sedimentary rocks represent altered 

 igneous material, from which salts have been leached into the ocean, 

 and to which oxygen, water and carbon dioxide have been added 

 from the atmosphere. For these changes, corrections can be applied, 

 and their magnitude and effect, as will be shown later, is surprisingly 

 small. The thin film of organic matter upon the surface of the earth 

 can be neglected altogether. In comparison with the ten-mile thick- 

 ness of rock below it, its quantity is too small to be considered. 

 Even beds of coal are negligible, for their volume also is relatively 

 insignificant. Practically, we have to consider at first, only ten 

 miles of igneous rock ; which, when large enough areas are studied, 

 averages much alike in composition all over the globe. This point 

 was established in my original memoir, in which groups of analyses, 

 representing rocks from dift'erent regions, were compared. The 

 essential uniformity of the averages was unmistakable, and it has 

 been still further emphasized by later computations by others as well 

 as by myself. The following averages are now available for com- 

 parison. 



A. J\ly original average of 880 analyses, of which 207 w^ere 

 made in the laboratory of the U. S. Geological Survey and 673 were 

 collected from other sources. Many of these analyses were in- 

 complete. 



B. The average of 680 analyses from the records of the Survey 

 laboratories, plus some hundreds of determinations of silica, lime 

 and alkalies. The Survey data up to January i, 1897. 



C. The average of 830 analyses from the Survey records, plus 

 some partial determinations. The Survey data up to January i, 

 1900. 



D. An average of all the analyses, partial or complete, made 

 up to January i, 1904, in the laboratories of the Survey.^ 



E. An average, computed by A. Marker,- of 397 analyses of 



^ See U. S. Geological Surv. Bull. 228, p. 17, for details. 

 ^ Geol. Mag., 1899, p. 220. 



PROC. AMER. PHIL. SOC, XLV. l82B, PRINTED JUNE 25, I906. 



