570 CROSS, IDDINGS, PIRSSON, WASHINGTON 



ment in minettes and absence from certain basanites. And with 

 this difference in biotite there is a variation in olivine, hypers- 

 thene, magnetite and other minerals within the rocks mentioned. 

 Other notable illustrations of different mineral development 

 in chemically similar rocks are the madupite of Wyoming 1 and 

 venanzite of Italy, 2 in the latter rock melilite and olivine appear- 

 ing instead of pyroxene and phlogopite in the madupite ; also the 

 nephelite-syenite of Beemerville, N.J., 3 and the leucite-phonolite 

 of Bracciano. 4 Indeed, instances of the same kind are well known 

 to all, and are constantly increasing in number. It is therefore 

 indisputable that magmas of identical chemical character may 

 and do solidify as very different mineral aggregates, it being 

 also a possibility that they form on solidification no minerals at 

 all, or that they crystallize only in part. 



Standard mineral composition. — Whether vitreous or crys- 

 talline, all igneous rocks may be correlated by considering what 

 mineral combinations may be developed from their magmas if 

 completely crystallized. But since several mineral combinations 

 are possible for most magmas, it is advisable to select one of 

 these combinations as the standard of comparison. And for 

 uniformity and simplicity it is necessary to select the same one 

 for all rocks having like chemical composition. This may be 

 termed the standard mi?ieral composition, which may or may not 

 correspond to the actual mineral compositio?i. 



Before presenting the reasons for selecting certain minerals 

 as those best adapted for a chemico-mineralogical classification 

 of igneous rocks, let us consider the important rock-making 

 minerals from the general standpoint of their chemical compo- 

 sition. They may be arranged in several groups chiefly 



1 Cross, W., " Igneous Rocks of the Leucite Hills, etc." Am. Jour. Sci., Vol. IV 

 (1897), pp. 115-141- 



2 Sabatini, V. I Vulcani di S. Venanzo. Rivista di Min. e Crist., Vol. XXII 

 Padova, 1899, pp. 1-12. Cf. Rosenbusch. Sb. Berl. Ak. (1899) p. 113. 



3 Bull, 150, U.S. Geol. Surv. (Washington, 1898), p. 209. 



4 Washington, H. S., "Italian Petrographical Sketches," Jour. Geol., Vol. V 

 (1897), pp. 43,49. 



