Pirsson — Petvographic Province of Central Montana. 43 



worth while, however, to give all these analyses,* for some of 

 them from a given area, for our purposes, would be merely 

 repetitions of one another, and in this case only one is selected 

 to represent this variety of magma. Only the essential ele- 

 ments are given and consequently the summations are omitted. 

 In examining these tables of analyses the first thing that is evident 

 is that in general, high silica, alumina and alkalies go together* 

 and are opposed to lime, iron and magnesia. This is of course 

 merely a general truth applicable to all igneous rocks and not 

 a special character of the province. The special and most 

 obvious feature which distinguishes this district is in the rela- 

 tion of alkalies to one another and to silica. The potash domi- 

 nates over the soda. 



General law of the province. — Definitely stated it is this ; 

 The petrographic province of central Montana is characterized 

 by the fact that in the most siliceous magmas the percentages 

 of potash and soda are about equal; with deer easijig silica 

 and increasing lime, iron and magnesia, the potash relatively 

 increases over the soda, until in the least siliceous magmas it 

 strongly dominates. An inspection of the tables will show 

 that there are but a few partial exceptions to this law in the 

 40 analyses given, and since all which are exceptions are given, 

 the 18 omitted analyses would merely add the weight of 

 additional figures to the truth of the law. 



Application to the region. — It will be of interest now to 

 examine this more in detail with respect to the various moun- 

 tain groups. The Castle Mountains lie on the extreme southern 

 border of the province ; their next neighbor to the south is the 

 Crazy Mountains group, and an examination of analyses from 

 that districtf shows that in the magmas soda strongly dominates 

 the potash throughout the series. The writer has already shown J 

 that the general Castle magma was one of a very salic charac- 

 ter, in fact that of a granite, and that femic rocks play but a 

 small role. Thus in the siliceous types we see the influence 

 of the nearby Crazy Mountains' magma ; the soda here slightly 

 dominates the potash ; we are on the edge of the province and 

 the rocks are transitional. The relation to it is seen however 

 in the most femic type, since here potash dominates the soda. 

 As we go northward from here into the province the Little 

 Belt rocks came next and its characters become more evident. 

 In percentages potash begins to rule even in the siliceous types 

 and in the extreme femic types this peculiarity is strongly 

 marked. Only two exceptions are noted, both of which are 

 given and both of which are narrow dikes. Their exceptional 



* They mav be found in the works previously cited. 

 f Bull. U. S. Geol. Surv. 168, pp. 120-124, 1900. 



% Geology of the Castle Mountain Mining District, Bull. U. S. Geol. Surv. 

 139, p. 138. 



