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SCIENCE PROGRESS 



and those of some other workers, point also to the same sig- 

 nificant conclusion. In the case of basic rocks, however, the 

 applicability of this generalisation is doubtful, and more analyses 

 are required before it can be extended to that class of rocks. It 

 is interesting to observe, in this connection, that volcanic rocks 

 contain more soda and more silica than the corresponding 

 plutonic rocks. With regard to potash, the volcanic rocks are, 

 if anything, poorer rather than richer, but the differences are in 

 this case variable and alternating. The following table is com- 

 piled from the average analyses of rock types collected by 

 R. Daly 1 and A. N. Winchell. 2 The distinction to which atten- 

 tion is drawn appears to hold too consistently to be assignable 

 to the particular choice of material ; rather is it an expression 

 of a real difference due to differentiation. 



Comparison of Volcanic and Plutonic Rock Types 



1 R. Daly, Proc. Am. Ac. Arts, and Sci., vol. xlv. p. 211, 1910. 

 Igneous Rocks mid their Origin, pp. 13-39, New York, 19 14. 

 ■ A. N. Winchell, Journ. Geo/., vol. xxi. p. 208, 191 3. 



See also 



