SYSTEM A TIC PETROGRAPHY 455 



The morphological characteristic of rocks chosen by von 

 Lasaulx as most applicable to the systematic subdivision of the 

 classes mentioned was the degree or distinctness of crystallinity, 

 which is surely the most variable of all their properties, and 

 hence least adapted to the formation of well-defined groups. 

 The major divisions formed by von Lasaulx are as follows : 



Simple Rocks : 



A. Non-crystalline (amorphous) or semi-crystalline. 



B. Crystalline granular. 



a. Really simple. 



b. Rocks forming transitions to Composite group through the appear- 



ance of vicarious constituents (<?.£•., amphibolite, serpentine, etc.). 

 Composite Rocks : 



A. Massive. 



a. Amorphous, glassy (obsidian, etc.). 



b. Semi-crystalline (including vitrophyres). 



c. Crystalline. 



1. With abundant glassy base (basalt, etc.). 



2. With microaphanitic more or less individualized groundmass. 

 aa. Groundmass alone (felsite, etc.). 



bb. True porphyries (felsite-porphyry, etc.). 



3. Rocks which are almost completely crystalline, mainly pseudo- 

 porphyritic, etc. (phonolite, hornblende-andesite, etc.). 



4. Crystalline granular. 



aa. Feldspathic (granite, etc.). 



bb. Non-feldspathic (Greisen, eclogite, etc.). 



B. Stratified Rocks : 



a. Feldspathic (gneiss, etc.). 



b. Non-feldspathic (mica-schist, etc.). 

 Clastic Rocks : 



A. Semi-clastic (clay slate, kaolin, tuff, etc.). 



B. Purely clastic. 



Mineral composition is applied as a factor to produce the 

 commonly recognized rock types within these groups. It will 

 be noted that many of the divisions above mentioned are not 

 only quite indefinite but they are, also, inconsequent. Thus, 

 three out of four divisions of crystalline massive rocks are but 

 partly crystalline. 



Meteorites are described by von Lasaulx in an appendix. 



