268 L. V. Pirsson — Classification of Igneous Rocks. 



of very similar composition and texture. This was based 

 on historical grounds; the older rocks are more likely 

 to have suffered various modifications which cause them 

 to appear somewhat differently from fresh recent ones of 

 similar nature. Such differences were thought to be 

 fundamental in character, to belong to the period at 

 which the rocks were formed, and to thus deserve recog- 

 nition by giving such rocks an especial set of names. 

 Since it has been perceived, however, that these charac- 

 ters have been superposed on the rocks, and are therefore 

 secondary in nature, this distinction has lost its force and 

 has practically been given up, even in Germany, though 

 the names have been in large measure retained, as a 

 matter of convenience, to denote rocks which present 

 these characters, without regard to their geologic age. 

 This historical usage, which never obtained much cur- 

 rency in America and Great Britain, the student should 

 note, in order that he may understand the literature which 

 he may consult, particularly the older part of it, and 

 appreciate the meaning of these terms when he meets 

 them. They will be mentioned in appropriate places. 

 Returning now to our classification, we may illustrate its 



Table No. 1. Illustrating Qualitative Classification of Igneous Rocks 





Predominant Alkalic Feldspar 



Predominant Soda-lime Feldspar 



No Feldspar 



Even- 

 granular. 



With Quartz 



No Quartz 



With Ne- 

 phelite 



With Horn- 

 blende 



With Py- 

 roxene 



With Ne- 

 phelite 







Granite 



Syenite 



Nephelite- 

 Syenite 



Diorite 



Gabbro & 

 Norite 



Theralite 



Peridotite 





Pre-Tertiary 



Porphy- 

 ritic, 

 dense, 

 glassy 



Granite- 

 Porphyry 



Quartz- 

 Porphyry 



Syenite- Neph. Syen. 

 Porphyry Porphyry 



Kerato- 



phyre 



Diorite- 

 Porphyrite 



Quartz 

 Porphyrite 



Diabase. 



Augite- 

 Porphyrite 









Tertiary and Recent 



Dense, 



glassy, 



porphyritic 



Rhyolite 



Trachyte 



Phonolite 



Dacite & 

 Andesite 



Basalt 



Basanite 



Augitite 



