656 DESCEIPTIVE GEOLOGY. 



the present topography; that is, tliey are generally inclined toward the 

 centre of the basin, which lies between Shoshone Pass and Boundary Peak, 

 called Soldier's Spring Valley. In estimating the thickness of these rhyolite 

 flows, by crossing them on a northeast line from the ridge which bounds 

 Antimony Canon on the north, one can reckon a thickness of over 6,000 

 feet of conformably superimposed beds of rhyolitic lavas. While these 

 beds, which vary in thickness from fifty to a few hundred feet, are all of 

 different external characters, there seems to be a certain sequence, which 

 vf ould indicate that there may be a repetition in the series caused by faulting, 

 so that this estimate cannot be definitely relied upon for their actual thick- 

 ness. They have been very extensively eroded, and are in general of some- 

 what earthy character, so that they are easily acted upon by the forces of 

 abrasion. 



Shoshone Pass is cut along the line of strike of these beds, and at its 

 highest point is only a few hundred feet above the neighboring valleys. 

 Along the pass are numerous springs of fresh, pure water, the most promi- 

 nent group of which is called Shoshone Springs. Among the low hills to the 

 north of Shoshone Springs are a few which can be prominently distinguished 

 from a great distance by their dark color, in strong contrast to the prevailing 

 light colors of the rhyolites. They are found to be formed of beds of dark- 

 blue limestone, which are bent into a narrow anticlinal fold, whose axis has 

 a north and south direction, and of which the eastern member stands almost 

 vertical, while the western' beds slope off at an angle of 20° to 25°. The 

 aggregate thickness of these limestones could not be determined, as the out- 

 crops are isolated by intervening accumulations of Quaternary material ; but 

 an exposure of over 1 ,000 feet of limestone is shown, with interstratified beds 

 of reddish,. clayey, and arenaceous shales. The limestones have been very 

 considerably metamorphosed by the action of the rhyolitic overflows, and are 

 very black and hard on their weathered surface, often covered with a thick, 

 black coating, rich in iron. In one place, a considerable portion of the lime- 

 stone has been changed into a white crystalline calcite, which is concre- 

 tionary and bounded by a coating of carbonate of iron about an inch and 

 a half in thickness. 



In the hills to the south of the spring is found an outcrop of conglom- 



