CHAPTER II. 



THE INTRUSIVE ROCKS OF THE GALLATIN MOUNTAINS, 

 BUNSEN PEAK, AND MOUNT EVERTS. 



By Joseph Paxson Iddings. 



Having described the occurrence of the igneous rocks that have been 

 intruded within the sedimentary beds of the Gallatin Mountains, or have 

 been thrown over their surface, so far as their occurrence is related to the 

 history of the dynamic events that brought about the present structure 

 and topography of the range, we may now describe their petrographical 

 characters in relation to the mode of their occurrence, with special reference 

 to the size of the various bodies of rock and their geological position. 



From what has already been shown as to the relative age of the 

 different intrusions, it will be proper to consider them in the following 

 order: Indian Creek laccolith; Holmes bysmalith and connected outliers; 

 Bighorn Pass sheet; Gray Mountain intrusive and connected sheets; 

 Electric Peak stock and dikes, together with the extrusive breccias and 

 intrusive dikes of Sepulchre Mountain; and the breccias west and south of 

 the Gallatin Range. In this connection may also be described the Bunsen 

 Peak intrusive and the intrusive sheets in Mount Everts. 



INDIAN CREEK LACCOLITH. 



HORNBLENDE-MICA-ANDESITE-PORPHYRY. 



The rock constituting this laccolith and its two sheet-like apophyses to 

 the south is an intrusive mass, quite uniform in mineral composition 

 throughout its whole extent. It exhibits a limited variability in texture 

 and habit, ranging from those of a compact aphanitic or lithoidal lava to 

 those of a minutely crystalline porphyry-like rock. Its predominant min- 

 eral constituents are lime-soda feldspar, hornblende, and biotite, with a 

 small amount of magnetite and, in the coarser-grained forms, quartz. For 

 this reason it may be called an andesite-porphyry with andesite facies. Its 



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