38 Pii^sson — Petrographic Province of Central Montana. 



details the reader is referred to the series of memoirs upon 

 them whose list is given upon a foregoing page. 



The evidences of consanguinity are to be seen in two ways, 

 in certain mineral peculiarities and in the chemical composition 

 of the magmas, the first being dependent upon the second 

 in conjunction with the physical conditions attendant upon 

 crystallization. 



Map of Central Montana showing arrangement of mountain groups in 

 petrographic province. 



Consanguinity s/wion by minerals. 



Augite. — One of the most marked features in regard to the 

 mineral composition of this composite geographical rock fam- 

 ily is to be seen in the augite. This has been already pointed 

 out by Iddings,* but its application to this province is worthy 

 of special mention. 



The augite is of a distinct green color, very rarely pleo- 

 chroic. It varies from very pale to a deep green. Brown or 

 purplish augites are rare. They do occur in some of the 

 lamprophyric dikes and flows but are exceptional, so that in a 

 great preponderance of the rocks the green augite distinctly 

 rules. Moreover this applies through the whole series from 

 *Op. cit., p. 131. 



