ROCK CLASSIFICATION 99 



reference to these two factors. A third factor may be expressed 

 by inserting a number at the locus in the diagram of each rock 

 analysis which shall indicate some other molecular ratio, such 

 as that of the approximate feldspar 



/ AL 2 Q ? -{Na 2 + K- 2 Q) \ 



\ 2(Na. 2 + K % 0) J 



Or in another case the molecular ratio between the ferrous 

 oxide, magnesia, and the lime not calculated with the anorthite 

 molecule, and the silica. 



( FeO+MgO+ CaO- (A/ 2 6> 3 - [IVa 2 Q+K 2 Q]) \ 

 \ Si0 2 ~) 



The comparison by the method just indicated of 928 chem- 

 ical analyses of igneous rocks has given rise to the accompany- 

 ing diagrams. The analyses were selected with care in order to 

 avoid those of much altered rocks, or those likely to be untrust- 

 worthy. The greater part have been made by chemists of the 

 U. S. Geological Survey, which has furnished the most consid- 

 erable contribution to the knowledge of the chemistry of rocks 

 ever made by any organization. The analyses include those of 

 all known kinds of igneous rocks that have been analyzed, so 

 far as the writer has encountered them. 1 



Diagram 1 shows the distribution of all the analyses, the 

 black spots representing those in which soda is more than twice 

 the potash molecularly ; the red spots those in which it is less 

 than twice the potash. Diagram 2 is for all those in which the 



1 i • r 1 1 • 1 Na~0 



molecular ratio of soda to potash is greater than 2 , ** >2. 



K 2 O 



Diagram 3 is for all those in which this ratio is less than 



Na.c.0 _ . , .... 



2, < 2. Diagram 4 is tor those analyses in which 



• • , 11 Na 2 ,\ „ 



this ratio is equal to or less than 1 , * ji^ 1 - One ot 



K 2 U 



1 The analyses have been taken from the following sources : Analyses of Rocks 

 and Analytical Methods, Bull. 148, U. S. Geol. Survey, 1897. Analyses in papers 

 published by Pirsson and Weed. Roth's tables of rock analyses. Brogger's pub- 

 lications, and many others. 



