Pirsson — Petrographic Province of Central Montana. 37 



dikes and sheets surrounding the main stocks of granular 

 rocks, only serve to awaken general interest as to the character 

 and relations of these rock masses, and it is to be greatly hoped 

 that Professor Wolff will be able to continue his studies upon 

 this interesting material and publish his results for the benefit 

 of petrographers and for the understanding of the region. In 

 the Bearpaw Mountains the researches of the writer upon the 

 material collected during a hurried trip through them by Mr. 

 Weed, which brought out such a variety of novel types of 

 alkalic rocks, can only serve to demonstrate that this relatively 

 large area must afford a fruitful field of study in the future ; 

 one whose complete investigation will add much to our knowl- 

 edge of theoretic petrology and yield many interesting rock 

 types. 



The same must in large measure be true of the Sweet Grass 

 Hills. The material studied by the writer gave types much 

 like those of the Judith Mountains with hints of alkalic ones 

 accompanying them, and the appearance of some specimens 

 forwarded to Mr. Weed would seem to indicate that rocks of 

 tinguoid habit occur there. Adding these facts to Dr. Daw- 

 son's* descriptions, it would seem as if they might consist 

 largely of laccoliths probably with accompanying sheets and 

 dikes similar in character and in rocks to those of the Judith 

 and Little Rocky Mountains and the Black Hills. 



Definition of the province. 



That part of this great region which has been studied by the 

 writer, and with which he is therefore most familiar, lies in the 

 center of Montana and embraces as its foci of igneous activity 

 the Castle, Little Belt, Judith, Highwood, Bearpaw and Lit- 

 tle Rocky Mountains. Since the general reader cannot be 

 expected to be familiar with the geography of this region and 

 the disposition of these groups, their arrangement with respect 

 to one another and to the main chains of the Rocky Moun- 

 tains is shown on the accompanying sketch map. It will there 

 be seen that they lie in a roughly oval area stretching from 

 the northeast towards the southwest, about 150 miles long by 

 about 100 broad, in the middle of Montana and shown on the 

 map by the dotted line. It is this region which it is here pro- 

 posed to define as the petrographic province of central Mon- 

 tana ; the consanguinity and general family relations of whose 

 rocks it is intended to describe. 



This paper then may be considered as a general summation 

 along the line just mentioned of the work of the writer on 

 these different mountain groups, presenting the broad petro- 

 logic features they possess in common. For the separate 



*Rep. Canadian Geol. Surv., 1882-4, Pt. C, pp. 16 and 45. 



