572 CROSS, IDDINGS, P IRS SON, WASHINGTON 



established affinities or associations, and by successively sub- 

 dividing these groups by subordinate chemical differences or 

 quantitative values. For this purpose it is necessary to assemble 

 all rock-making minerals into two chemically distinguished 

 groups. 



These have been made by uniting quartz, corundum and 

 zircon with the aluminous non-ferromagnesian minerals — feld- 

 spars, feldspathoids and muscovite — in one group, and by 

 placing the non-aluminous ferromagnesian and calcic minerals 



— hypersthene, diopside, acmite, olivine, and akermanite — with 

 the non-siliceous and non-aluminous minerals, and titanosilicates, 



— magnetite, hematite, ilmenite, apatite, etc. — in the other 

 group. 



This leaves the aluminous ferromagnesian minerals to be 

 treated in another manner. The reasons for this separation of 

 the minerals, biotite, amphibole and augite, are discussed at 

 length in a later part of this article, but it may be said here that 

 their variable composition and occurrence, together with the fact 

 that they may be considered as mixtures of aluminous and non- 

 aluminous molecules, make it advisable to defer their introduction 

 into the system of classification until the actual mode of crys- 

 tallization of the rocks is taken into account. Since it is possible 

 that a magma of any given chemical composition may crystallize 

 without the development of these minerals, and since the 

 chemico-mineralogical expression of igneous magmas is greatly 

 simplified by not considering these minerals until the particular 

 crystallization of the magma is to be expressed, we are justified 

 in omitting them from the two groups of minerals which are to 

 be employed in determining the standard mineral composition 

 of an igneous rock. These two groups of standard minerals are: 



GROUP I : SALIC MINERALS. 



Quartz, Si0 2 Q 



Zircon, Zr0 2 .Si0 2 -------- Z 



Corundum, A1 2 3 --------- C 



Orthoclase, K 2 O.Al 2 3 .6Si0 2 or) 



Albite, Na 2 O.Al 2 3 .6Si0 2 - - - ab \ F 



Anorthite, CaO.Al 2 3 .2Si0 2 an) 



