CLASSIFICATION OF IGNEOUS ROCKS 



39 



. Orders. — Each of the first three classes is divided into orders 

 (Fig. 3) according to the Ab-An ratio in the plagioclase. The 

 division points are AbiooAbo, AbggAns, AbgoAnso, Abs An<,5, AboAn.oo. 

 There are thus formed, for each class, four double triangles (Fig. 4), 

 in each of which three angles represent (i) quartz (Qu), (2) all 

 feldspars except plagioclase (Kf), and (3) the feldspathoids (Folds). 



Adamelllte 



Orthoeyenlte 

 Kf 



Fig. I. — A proposed change in family divisions 



Jrtho- 

 iranlte 



K-Grar Ite 



Granite 



Quartz 



Norma 11 Gran. 



Quart z-monzonlte 



Granodlorlte 



Grar.odlorl ^e 



-monzohltc 



Quartz-nonzonlte Gi anodlO'lte Qv-Dlor. 



Granodloi Ite 



Soda-ltme-Gr JQu-Monfeonlte 



Qu-Dlcrlte 



Granodlorlte 



Quart z- 

 Dlorlte 



Qu-Dlor. 



QuTDlor 



Fig. 2. — Variations in the usage of names in the granite quartz-diorite series: 

 A, divisions used in the present classification; B, divisions suggested (cf. Fig. i); 

 C, Lindgren's original divisions; D and E, divisions as used by certain other 

 petrographers. The right ordinate is orthoclase, the left, plagioclase. 



The remaining angle (Flag) represents albite (Naf), oligoclase to 

 andesine (CaiVaf), labradorite to bytownite (NaCaf), or anorthite 

 (Caf), and these constitute the basis for the separation into Orders 

 I, 2, 3, and 4. 



In Class 4, owing to the absence of light constituents, it is neces- 

 sary to make the subdivisions on a different basis. In this class. 



