640 CH OSS, ID DINGS, P1RSS0N, WASHINGTON 



diorite of d'Aubuisson (1819) as defined by von Leonhard 

 (1823). It will include the less feldspathic diorites and horn- 

 blende-gabbros. 



4. Gabbro — all granular igneous rocks with dominant pyrox- 

 ene and subordinate feldspar of any kind, with or without horn- 

 blende and mica. Essentially the gabbro of von Leonhard. Since 

 it is not possible to identify pyroxene as distinct from hornblende 

 in many cases, megascopically, it will probably happen that all 

 of those rocks which can be clearly seen to contain dominant 

 hornblende will be called diorite, and all doubtful ones will be 

 grouped with the distinctly pyroxenic gabbros. These rocks 

 will include the less feldspathic gabbros and norites, and some 

 diorites. 



5. Peridotite, pyroxenite, and hornblendite — all granular igneous 

 rocks composed almost completely of olivine, pyroxene, or 

 hornblende, in variable proportions, with little or no feldspar. 

 These names are to be applied as at present. 



Other names in common use, which can be applied without 

 confusion upon the basis of purely megascopical determination, 

 may be used. 



II. Aphanites — lithoidal, aphanitic rocks. These may be 

 non-porphyritic or porphyritic, th e aphanitic character being 

 confined to the groundmass. 



A. Non-porphyritic forms, having no recognizable mineral con- 

 stituents, must be subdivided, if at all, upon the basis of color, 

 luster, or other physical properties. The early distinctions were 

 in reality on a basis of color, and were two : 



1. Felsite (Gerhard, 1814), Phonolite (Klaproth, 1801), Petro- 

 silex of the French geologists. Felsite includes all aphanitic 

 igneous rocks that are non-porphyritic and are light-colored, in 

 various tones, and with various lusters other than vitreous. 

 They include lithoidite (von Richthoven, i860), or lithoidal 

 rhyolite, non-porphyritic trachite, and phonolite, and the lighter 

 colored non-porphyritic andesites, latites, etc. 



2. Basalt — all dark-colored, aphanitic, igneous rocks without 

 phenocrysts. This will include the dark-colored andesites, non- 



