198 GEOLOGY OP THE YELLOWSTONE NATIONAL PAEK. 



in the valley of Snake River, similar basalts have broken out. Along Fox 

 Creek the fault is not so readily traced, owing- to the broad benches of 

 morainal drift and the occurrence of basic breccia which lie on both sides 

 of the displacement. 



These breccias are well exposed in the more recent cuts along Fox 

 Creek, and in every way resemble those found on Two Ocean Plateau. 

 They are andesitic breccias, firmly held together by finer material, generally 

 colored red from the oxidation of the ferruginous material. West of Fox 

 Creek, on the slope of Big Game Ridge, is an outcrop of rhyolite, interesting 

 from the fact of its being the most easterly exposure of that rock west of 

 Two Ocean Plateau. This rhyolite is evidently a remnant of a much larger 

 body of purplish-gray, normal rock, like most of that seen on Flat Mountain. 

 It disintegrates readily into rhyolitic gravel, in rather strong contrast to the 

 weathered and crumbling sandstone. 



Along Mink Creek the fault contact between the Cretaceous and Car- 

 boniferous is strikingly shown, a great thickness of both rocks being well 

 developed, Mink Creek having cut deeply into the Madison limestone. 

 Pacific Creek below Mink Creek also marks the fault line until it passes 

 beyond the limits of the mapped area. 



pacific creek. — This stream, which has its source along the continental 

 divide, has cut for itself a broad channel on both sides of the older valley. 

 The breccias have been worn away, exposing a body several hundred feet 

 in thickness of light-colored crystalline limestone, characteristic of the upper 

 members of the Madison limestone. The upper beds are highly siliceous 

 and may possibly belong to the Quadrant quartzites, although no such heavy 

 masses of siliceous beds were recognized as to warrant a reference to the 

 overlying formation. An anticline in the limestone crosses Pacific Creek 

 in a northwest-southeast direction, the beds dipping steeply on both sides 

 of the fold. In the valley the limestone lies nearly horizontal. 



According to Prof. J. P. Iddings, both the Teton and the Ellis formations 

 are well exposed, overlying the Madison limestone, just south of the limits 

 of the area mapped. The red beds have a development fully 600 feet in 

 thickness, and are followed by a gray limestone carrying- a bed of white 

 gypsum 5 feet in thickness. Overlying the latter comes a gray fissile lime- 

 stone, followed by massive beds of limestone characterized by an abundance 

 of fossils. From these beds were collected the following species, which 



