232 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



EXTENT OF EROSION. 



As already stated, the volcanic ejectamenta were thrown over the sur- 

 face of greatly eroded sedimentary rocks. It appears from a study of the 

 adjacent region that an apparently conformable series of deposits from 

 Cambrian to the Laramie of the Cretaceous had been greatly dislocated and 

 faulted, and in places entirely eroded down to the crystalline schists, before 

 the volcanic lavas of Crandall Basin were erupted, thus representing a period 

 of great orographic movement and denudation. It becomes equally evident 

 from a study of some of the areas of volcanic rocks that, after the earlier of 

 these rocks were extravasated, both orographic movement and denudation 

 took place on a grand scale. The region of Electric Peak and Sepulchre 

 Mountain exhibits the extent of the faulting- which cut in two that andesitic 

 volcano. But in the region of Crandall Basin, which seems to have 

 escaped serious disturbance since the accumulation of the basaltic lava, we 

 may discover a measure of the erosion which has affected this portion of 

 the country subsequent to the completion of this volcano, which must have 

 been active in upper Miocene time. 1 However, it should be confessed at 

 the outset that all such calculations must be of the crudest and most general 

 character. 



A consideration of the geological structure which has been briefly 

 sketched, and which has been indicated on the map and in three vertical 

 sections across the district through the gabbro core, leads to interesting con- 

 clusions. The profile sections (PI. XXXII) are drawn to natural scale and 

 exhibit the steepness of some of the mountains. The first passes through 

 the core in a direction N. 24° E., and cuts Pollux, Parker, and Indian 

 peaks, the narrow ridge south of Hurricane Mesa, and the low hills north, 

 ending in the gneiss on Clark Fork. The second lies N. 20° 30' W., pass- 

 ing from the divide between Crandall and Sunlight basins, through the gab- 

 bro core and Index Peak, to the gneiss at the head of Soda Butte Creek. 

 The third passes S. 77° 30' E., from Druid Peak across the valleys of Soda 

 Butte and Cache creeks, cutting the ridge of The Thunderer where it is 

 narrowest, and, traversing nearly the whole length of Hurricane Mesa, 

 passes through the summit of Windy Mountain. 



The chaotic accumulations of scoriaceous breccia and the occurrences of 

 steeply dipping beds and lava flows throughout the area of Crandall Basin 



1 Hague, Arnold, loc. cit., p. 452, 



