Pirsson — Petrographic Province of Central Montana. 45 



So far as is known, the Sweet Grass Hills off to the northwest 

 agree with the last two groups, but the absolute confirmation of 

 this must await future exploration and study. Moreover 

 they are somewhat outside of the general area under discussion. 

 In this connection however there might be mentioned Big 

 Snowy Mountain, south of the Judith Mountains. This is evi- 

 dently a large laccolithic uplift, and from the Judith Mountains 

 the heavy white Carboniferous limestones dipping away from it 

 are clearly seen. Intelligent mining prospectors, who have 

 searched the mountain for ore deposits, have assured me that it is 

 all limestone on top and that no porphyry is exposed. The lacco- 

 lithic roof has evidently not yet been eroded away, but consid- 

 ering all the facts of structure and occurrence in this province, 

 there can scarcely be reasonable doubt that a concealed body 

 of feldspathic porphyry lies underneath the limestone dome. 



Geographical Arrangement of JIagmas. 



From what has been already said, it is now evident that there 

 is a rather orderly arrangement of magmas in the province. 

 Around the outer edge they tend to be strongly siliceous, low 

 in lime, iron and magnesia, and with the percentages of soda 

 about equal to those of potash, and these magmas have usually 

 marked their upward movement and intrusion by the formation 

 of laccoliths. One can say in truth that the boundary of the 

 province on the south, southeast, east, north and if it be extended 

 to include the Sweet Grass Hills, on the northwest, is defined 

 by laccoliths or groups of laccoliths of a rather definite type of 

 magma. On the west the boundary is not yet well known and 

 is perhaps not so clearly defined. At all events, in this direc- 

 tion it is eventually cut off by the main ranges of the Eocky 

 Mountains, whose magmas, so far as we know them, are of a quite 

 different character, belonging in fact to the granite-diorite series 

 whose surface equivalents are rhyolites, andesites and basalts. 



On the border line of the province thus defined femic types 

 are rare or wanting. When they appear, however, they tend to 

 assume the regional character and potash rises. As we approach 

 the center of the province they become more numerous and of 

 larger volume and as silica sinks the potash rises. This is 

 shown by the occurrences of inonzonite and shonkinite in the 

 Little Belts and on its northern edge. Finally, in the central 

 portion of the province in the Highwoods, on the Missouri 

 river and in part of the Bearpaws the magmas are distinctly 

 femic and rocks rich in ferro-magnesium components form the 

 largest masses, are most numerous in occurrence and distinctly 

 rule. There is still a recurrence of salic types, but they are of 

 small volume and of diminished importance. 



