92 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



It is probable that, instead of the broad bodies of intrusive andesite and 

 dacite represented on the map, there are a number of smaller bodies of 

 similar rocks intersecting one another, but the data at hand are insufficient 

 to enable their more accurate representation. 



In the northwestern spur of- the mountain the dikes are well marked, 

 from 5 to 25 feet wide, and not perfectly straight. Some of the intrusive 

 bodies carry inclosed masses and small fragments of black shale, and where 

 the fault plane traverses the massive igneous rock the latter has been crushed 

 into angular fragments, which are cemented together by particles of the 

 same rock, producing a crushed breccia, somewhat resembling the tuff- 

 breccia. 



The rhyolite that occurs over the breccia in Glen Creek Valley is part 

 of the great rhyolite sheet, and is of much more recent date, following the 

 faulting and erosion of the Electric Peak and Sepulchre Mountain masses. 



The accompanying map (PL XVI) shows in a simple manner the chief 

 geological features of a limited area embracing the rocks to be described. 

 The sedimentary terranes are colored according to the period in which they 

 were formed, embracing the Carboniferous, Juratrias, and Cretaceous. 

 The sheets of igmeous rocks intruded between the strata are not drawn 

 continuous, as they exist, owing to the fact that the data are insufficient. 

 They are more numerous than represented on the map, and are thinner. 

 The same is true of the dikes. 



Although the two mountains were at one time geologically connected 

 and the eruptive rocks were in a sense a geological unit, it will be conven- 

 ient and profitable to describe them separately at first, and afterwards to 

 consider their correlation. 



THE INTRUSIVE ROCKS IK" ELECTRIC PEAK. 



The intrusive rocks in Electric Peak, west of the fault, occurring in 

 the stock and its apophyses and in dikes, form a group of diorites and 

 diorite- and andesite-porphyries of variable composition and structure. 

 They grade into one another by transitions in composition and structure. 

 The coarse-grained granular rocks — diorites — occur almost wholly within 

 the stock and its larger apophyses, while the finer-grained porphyritic 

 rocks — porphyries — occur in the dikes and smaller apophyses, and in places 

 along the margin of the stock, in contact with sedimentarv rocks. 



The greater part of the stock is diorite, which varies in structure and 



