362 Weed and Pirsson — Bearpaw Mountains, Montana. 



most probable, or a later intrusion. In any case it is a differ- 

 entiated phase of the same magma. As seen in the table 

 below, its correspondent exists at Beaver Creek, but the one 

 corresponding to the syenite is wanting. It probably exists, 

 but was not observed in the hasty examination we have been 

 able to make. 



Quartz 

 Syenite. 



sio 3 67 ' 04 



bl ° I 68-23 



AlO 



25 



2 



FeO \ r69 



re ^ s ) 1-90 



j 15-2 

 ( 15-1 



FeO 



MgO 



CaO 



Na 2 





Monzorrite 





Syenite. 



"Yogoite." 



Shonkinite 



61-15 



54-42 



48-49 







52-81 



50-00 



15-07 



14-28 



12-29 







15-66 



9-87 



2-03 



3-32 



2-88 







3-06 



3-46 



2-25 



4-13 



5-77 







4-76 



5-01 



3-67 



6-12 



9-91 







4-99 



11-92 



4-61 



7-72 



9-65 







7-57 



8-31 



4-35 



344 



2-22 







3-60 



2-41 



4-50 



4-22 



4-96 







4-84 



5-02 



1-13 



•84 



1-75 



•54 



2-17 



•92 



4-09 

 5-30 



KO \ 5 " 10 



^ U ) 5-57 



The other types are clearly shown. In general a compari- 

 son of the twb localities shows the Beaver core as the more 

 alkaline of the two ; and the most important difference is that 

 at Beaver Creek ; the A1 2 3 and MgO show a much greater 

 degree of concentration and inverse variation than at Yogo. 

 This inverse variation is especially marked in the Beaver Creek 

 monzonite and shonkinite, which otherwise do not greatly 

 differ from one another. They thus furnish a most instructive 

 example of the fact that a nearly similar silica percentage may 

 exist in two types of rocks which differ greatly in appearance, 

 one with high Al 2 O s being clearly feldspathic and recalling 

 many medium diorites in appearance, while the other, with 

 high MgO, is dark, basic-looking, and with evidently prepon- 

 derant ferromagnesian minerals. 



There are a number of interesting dike rocks connected with 

 the Beaver stock ; these have been studied, and an account of 

 them, together with some other occurrences of rare types of 

 both intrusive and extrusive rocks in the Bearpaw Mts., will 

 be shortly published in a second paper in this Journal. 



Washington and New Haven, March, 1896. 



