SYSTEMATIC PETROGRAPHY 457 



occurrence, and involves no possible prejudice concerning 

 genetic relations." 



The controlling criteria in the further construction of Rosen- 

 busch's scheme will be apparent from the following partial 

 tabular statement : 



Massive rocks. 



A. Orthoclase rocks. 



a. Older. 



I. Quartzose. 



1. Granular = Granite Family. 



2. Porphyritic = Quartz porphyry Family. 



3. Vitreous = Felsite-Pitchstone Family. 



II. Quartz free (3 families similar to those above). 



b. Younger. 



I. Quartzose. 



1, 2. Granular or porphyritic — Liparite Family. 

 3. Vitreous = Family of the acid glasses. 

 II. Quartz free (with families as above). 



The other large groups are the following : 



B. Onhoclase-Nephelite, or Orthoclase-Leucite rocks. 



C. Plagioclase rocks. 



D. Plagioclase-Nephelite, or Plagioclase-Leucite rocks. 



E. Nephelite rocks. 



F. Leucite rocks. 



G. Non-Feldspathic rocks. Of the Non-feldspathic rocks Rosenbusch 



remarked 1 that they were all rich in olivine, and might therefore be 

 called Olivine rocks. 



Each of these groups has Older and Younger divisions, and, 

 within these, families, established in a manner similar to that 

 given for the orthoclase rocks. 



In this arrangement mi?ieral composition is used, as in Zirkel's 

 system. The age distinction is applied without discussion. 

 Texture is given a prominent role, and chemical composition is 

 not used. 



Fonque and Michel-Le'vy, i8yg. — The first effects of the 



microscopical study of rocks upon petrographic system in France 



may be seen in the Mincralogie micrographique \ by F. Fouque 



'"Sie sind sammtlich reich an Olivin, daher kann man sie kurz als Olivinge- 

 steine bezeichnert." 



