636 



PETROLOGICAL ABSTRACTS AND REVIEWS 



will publish such a set. From page 23 to the end various type rocks are 

 calculated and the relations between their mineral composition and their posi- 

 tion in the classification are discussed. The general divisions of the new 

 groups are as follows: 



The values of a have intervals of 3 between 30 and 7, from 6 to i the interval 

 is 2: the values of c, except the first, have intervals of 2. 

 The limiting values of the older rock types are: 



Rock Type 



Granite and Quartz-diorite 



Syenite and Diorite 



Essexite and Gabbro 



Mafic Essexite and Gabbro, 



Hornblendite and Pyroxenite 

 Dunite, Peridotite, and 



basic Hornblendite 



Nephelite-syenite 



Theralite and Shonkinite 



Alkalic feldspar-free Rocks . . . . 

 Anorthosite 



82.5-69 



74 -57 



60 -47 



46 -42 



41 -36 



67 -63 



S2> -49 



58 -46 



65 -50 



Limiting Values 



26-27 

 24-23 

 12-0 



2- o 

 26-13 

 12- 5 

 26- 3 

 II- i-S 



1.8 -1.2 

 I.I -0.9 



0.9 -o. 7 



0.7 -0.6 



0.6.-0.5 

 0.8 -0.7 

 0.65-0.5 

 o . 86-0 . 39 

 i.o -0.9 



Percentage 

 Si02 



63 

 65-51 

 53-46 



44-42 



40-35 

 59-54 

 46-43 

 50-39 

 57-44 



The first three groups included the acid, neutral, and basic igneous rocks. 

 According to Rosenbusch, the acid have 65 per cent, the neutral between 64 

 and 52 per cent, and the basic below 52 per cent of SiOz. The fourth and fifth 

 groups, with their extrusives are mafic, basic, and ultra basic differentiates of 

 the third group. The sixth group is an alkalic side-group of the second, the 

 seventh of the third. The eighth group includes rocks of very different mineral- 

 ogical character and can be regarded only as a basic side-group of the sixth 

 and seventh. The last group is cormected with the second and third but 

 belongs to the alkali series. 



