Weed and Pirsson — Bearpaw Mountains, Montana. 351 



Art. XLI. — The Bearpaw Mountains of Montana. First 

 Paper ; by Walter Harvey Weed and Louis V. Pirsson. 



[Continued from p. 301.] 



Beaver Creek Core. 



A few miles north of Bearpaw Peak the deeply trenched 

 valley of Beaver Creek cuts through an igneous center or 

 intrusive stock of granular rock which presents a highly inter- 

 esting example of an igneous mass which, intruded into sedi- 

 mentary strata, has there experienced a differentiation in place, 

 producing a gradational series of rock types. An excellent 

 illustration of this process, already described by the authors,* 

 occurs at Yogo Peak in the Little Belt Range, the front of the 

 Rocky Mountain Cordillera, 120 miles to the southward, and it 

 is interesting to find similar rock types repeated here. The 

 brief visit made to the Beaver stock did not permit of a 

 thorough study of the mutual relations of the different types 

 involved, but the most basic rocks were found near the periph- 

 ery of the intrusion, which is iu accord with the usual occur- 

 rence of such rocks. 



The locality is accessible by wagon road from Fort Assini- 

 boine to the prospect claims located on several small metal- 

 liferous veins occurring in the stock. The tract is one of 

 gently contoured, grassy summits trenched by abrupt and deep 

 drainage ways. The topographic relief affords no hint of the 

 presence of the massive rock, and the slopes are not scenically 

 attractive. The accompanying diagram (fig. 4) represents an 



E 

 Section through intruded stock at the head of Beaver Creek, Bearpaw 

 Mountains. 



ideal east-and-west section through the 'stock. The intrusion 

 is laccolithic in character, the sedimentary rocks dipping away 

 from it in every direction, as shown in the figure. The exposed 

 surface of the intrusion is about a mile across. The sedimen- 

 tary rocks are highly altered and metamorphosed in the con- 

 tact zone, and as usual these hardened strata resist erosive 

 agencies better than the granular rock forming the higher 



*Weed and Pirsson, Igneous Rocks of Yogo Peak, this Journal, 1, 467, 1895. 



