FA YAITE-IJOLITE SERIES OF MA GNET COVE 6 1 3 



approximate, but those of I, II, IV, and V may be very roughly 

 reckoned out, as below, that of IVa being Pirsson's calculation. 



In my former paper I discussed briefly the position of this 

 rock in classification, and provisionally put it with the shonki- 

 nites. At that time the mineralogical composition had not been 

 calculated, and this position was assigned to it because it resem- 

 bled Pirsson's shonkinites, except in the presence of nephelite 

 and of hornblende instead of biotite, and also because it came 

 under Rosenbusch's definition ' of these rocks, whose essential 

 features according to him, are the presence of abundant dark 

 minerals along with nephelite and orthoclase. As was also 

 remarked, it cannot be put with the essexites or theralites 

 (although chemically closely resembling these), on account of 

 the lack of plagioclase. 



In this connection it is of great interest to note the fact that, 

 in his latest description' of typical theralite, J. E. Wolff states 

 that there is nothing which can strictly be called soda-lime feld- 

 spar present. Indeed this fact is evident from a consideration 

 of the analyses by Hillebrand, published in the same place. 

 The name theralite, therefore, cannot be applicable to Wolff's 

 Montana rocks, or else its definition must be changed. 



^ ROSENBUSCH, Elemente der Gesteinslehre, p. 174, 1898. 

 ■> Bull. U. S. Geol. Surv., No. 150, p. 197, 1898. 



