10 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



SOUTH END HILLS. 



Commencing with the most southern end of the eastern part of the 

 ridge, where the sedimentary rocks begin to rise above the rhyolite plateau, 

 we find limestone exposed on the crest of the low ridge 3£ miles south of 

 Mount Holmes. The bedding is nearly horizontal, the dip being but 3° to 

 5° SW. throughout the greater part of the ridge. The character of the 

 limestone varies from shaly and fissile to massive beds, the highest strata 

 being mottled and banded, dark and light gray, and in places conglomeratic. 

 The lithological characters are like those of the Cambrian formations about 

 300 to 600 feet above the base of the series as it exists in the section north 

 of Crowfoot Ridge. 



At the southwestern extremity of the southern end of this ridge there 

 is a small exposure of coarse-grained gneiss, against which lies a bed of 

 fine-grained granular quartzite, about 50 feet thick, over which is light -gray 

 limestone in apparent conformity. The highly inclined position of these 

 beds, dipping 70° NE., with strike N. 50° W., and the nearly horizontal 

 position of the limestone a short distance east, indicate a fault between 

 these two sets of beds, which probably trends northwest and southeast, 

 with hade to northeast, and with a downthrow of not more than 500 feet. 

 The extension of the fault could not be traced on account of the covering- 

 of lava. 



The igneous rock intruded between the limestone and shales already 

 mentioned is the thin edge of one of the sheets of the Indian Creek 

 laccolith, and may be called andesite-porphyry. At the northern end of 

 the ridge in question it is exposed in a cliff 100 feet high, which is about the 

 thickness of the sheet in this place. Limestone is exposed beneath and 

 also above it. The intrusive sheet can be traced for several miles southward, 

 becoming thinner, until it is but 10 feet thick where last seen, before 

 disappearing beneath the rhyolite of the plateau. Above it, on the highest 

 portion of the ridge, two small dikes of andesite-porphyry traverse the 

 limestone across the axis of the ridge. A short distance to the southwest 

 there is a narrow vertical dike of similar rock, about 3 feet wide, trending 

 southwest and northeast. It rises slightly above the shaly surface of the 

 ground and exhibits two systems of inclined joints, forming rhombic hori- 

 zontal columns. Near the sides of the dike a third, horizontal joint splits 



