280 L, V. Pirsson — Classification of Igneous Rocks. 



and the plagioclases are normally those rich in lime, 

 from labradorite to anorthite. In E are the rocks that 

 contain no feldspar, or only negligible quantities of it. In 

 a chemical sense then we may say that the horizontal 



TABLE NO 3, PETROGRAPHIC 



DOMINANT 



A 

 Alkalic Feldspars 



! B 



Alkalic and Sodacalcic 

 Feldspars 



Characterized 

 by 



1. 



Felsie 



2. 

 Mafic 



3. 



Felsie 



4. 

 Mafic 



Quartz and 



Granite 



Granite Porp^y 

 Rhyolite 





Quartz Monzo- 



nite and Grano- 



diorite 



•Qtz. Mon. Porp'y 



Dellenite 





Feli>spars 





















Feldspars 



(little or no 

 quartz or lenad) 



10. 



Syenite 

 Syenite Porp^y 



Trachyte and 

 Bostonite 



11. 



Shonkinite 



Minette and Vo- 

 gesite 



12. 

 Monzonite 



Monzon. j^orp^y 



1 



Latite 



13. 

 Kentallenite 



Caniptonite 



Trachydolerite 



Feldspars and 



Lenads (felds- 



pathoirls) 



Felenites 19 



Neph -Syenite 

 Group. 



Neph. Syen. 



Porp'y 



Phonolite and 



Tinguaite 



20. 

 Malignite 



21. 

 Theralite 



(in part) 



22 

 Essexite 



(in part) 





Tephrites 





28. 



Lenites 



Ijolite Urtite 



Fergusite 



Group 



Neplielinite 



Leucitite 



(in part) 



29. 



Bekinkinite 



Missourite Gr. 



Neph. Leuc— & 

 Melilite Basalts. 

 N"ephelinite and 

 LeucititeCinpart) 

 Moiichiquite 



30. 



31. 



Lenads 

 (little or no feld- 







spar) 













arrangement 'represents a reciprocal relation between 

 alkalies and lime. 



In the vertical direction the chemical relation expressed 

 is that of silica (SiOo). "Wlien the silica is in excess 

 over that amount required to form feldspars, etc., we 

 have free quartz; as it diminishes quartz disappears; 

 next, there is not enough to turn all the alkalies and 



