216 



CLARKE— SOME GEOCHEMICAL STATISTICS. [April 20, 



ents of the rocks are considered, for the reason that the less con- 

 spicuous components have not been generally determined. They 

 will be separately discussed later. All the means have been recalcu- 

 lated to 100 per cent., and water, for obvious reasons, is excluded. 

 The methods for the determination of water are far from being 

 uniform, and the variations are so great as to obscure the essential 

 agreements between the other figures. The first table contains the 

 following averages. 



(A) The average of 248 "superior" analyses of igneous rocks 



selected by Washington from Roth's tables. See U. S. 

 Geol. Survey, Prof. Paper No. 28. Average computed by 

 the present writer. 



(B) The average of 536 British rocks, computed by Harker. 



" Tertiary Igneous Rocks of the Isle of Skye," Mem. Geol. 

 Survey United Kingdom, 1904, p. 416. 



(C) Washington's average of 1,811 rocks from all parts of the 



world. U. S. Geol. Survey, Prof. Paper No. 14, p. 106, 



(D) Loewinson-Lessing's estimate of a mean between an average 



granite and an average basalt. Inserted here for compari- 

 son with the other columns. 

 (£) The average of all the data relating to the composition of 

 igneous rocks contained in the laboratory records of the 

 U. S. Geological Survey. Computation by F. W. C. 



The general agreement is striking, and Loewinson-Lessing's esti- 

 mate fits well in with the others. The next table is devoted to the 



