44 Pirsson — Petrographio Province of Central Montana. 



position in the Little Belt series has been already discussed.* 

 In the siliceous rocks the dominance of potash as one goes 

 toward the center of the province sometimes expresses itself 

 more strongly ; thus in the granite porphyry of Wolf Butte 

 on the extreme northern edge of the Little Belts the relations 

 are K 2 : ~Na.fi = 4*4 : 3*3, as shown in an analysis not quoted 

 above. 



Next north beyond the Little Belts and near the center of 

 the province are the Highwoods, and here its characters reach 

 their highest development. The most siliceous type has equal 

 percentages of soda and potash ; there is only one occurrence, 

 the syenite of High wood peak ; in the others the potash strongly 

 dominates, increasing towards the femic end until in missourite 

 it is 4 to 1. There is only one exception in this group which 

 has been found, No. 28 in the table. It is the analcite basalt 

 of Highwood G-ap, occurring in narrow dikes. 



The general line through the province to the northward now 

 bends to the east toward the Bearpaws. Between the Bearpaws 

 and the Highwoods there is an occurrence of igneous rocks in 

 small stocks and dikes on the Missouri river. They have not 

 yet been throughly investigated, but the study of sections made 

 from specimens collected by Mi*. Weed shows them of femic 

 types like those occurring in the Highwoods. They are in fact 

 largely minettes of Highwood type, as previously mentioned, 

 and all their characters show that they closely conform to 

 the general law and that potash rules. 



Next beyond these come the Bearpaws, in which the general 

 law of the province strictly holds. It is to be noted that towards 

 the southern side the rocks are femic, and as we pass through 

 the group we "find on the northeast, on the edge of the province, 

 salic (quardofelic) types occurring in laccoliths again as in 

 Eagle Butte, the analysis of whose rock is shown in No. 32. 

 Again here as on the southern edge the soda rises until it 

 slightly exceeds potash. 



Southeast of here, defining the edge of the province in that 

 direction, are the Little Rockies, another laccolithic intrusion 

 of salic types. There is some variation and the rocks pass into 

 a tingnaitic phase. Exactly of the same character are the 

 Judith Mountains ; also a boundary group on the edge of the 

 province, of salic types running into tinguaites. There is as yet 

 only one analysis from each of these groups, one of a granite por- 

 phyry from the Little Rockies No. 39 and one of a tinguaite 

 (judithose) from the Judith Mountains. Thus they supple- 

 ment each other and the general law holds true, the potash 

 increasing as the silica falls. It is also to be noted that in 

 neither of these boundary groups do any femic types occur. 



* Petrography of the Little Belt Mts., 20th Ann. Rep. U. S. Geol. Surv., Pt. 

 iii, p. 576. 



