158 GEOLOGY AIS'D MINING INDUSTRY OF LEADVILLE. 



line of section is occupied by covered gaps, which could not be accurately 

 filled by offsets. The thickness of the sedimentary series from the Cam- 

 brian up to the top of the Cretaceous along this line has therefore been 

 assumed, in the ideal reconstruction of the surface, as that given by the sec- 

 tion of the Hayden Atlas, viz, 10,000 feet. The data obtained by Pro- 

 fessor Lakes afford no sufficient reasons for differing from tliis general con- 

 clusion. 



Four-Mile amphitheater. — The description iiow tums to tlic cxposurcs at 

 the head of the gulch and along the main crest of the range from Mount 

 Sherman to the head of Twelve-Mile Creek. The most striking of these 

 are shown in the sketch on Plate XVII, which represents the Horseshoe 

 and a portion of the Four-Mile amphitheater as seen from the junction of 

 the two branches of the creek. The shapes of these two amphitheaters dif- 

 fer characteristically, in accordance with the differing characters of the rock 

 out of which they have been carved. The erosion of the Four-]\Iile am- 

 phitheater, wdiich has been practically parallel with the strike of the beds, 

 has acted almost exclusively on the great ukiss of White Porphyry. Its 

 slopes are generally more rounded and largely composed of talus slopes of 

 angular fragments of this geologically brittle rock. In the bed of the stream 

 erosion has denuded a narrow strip of the Blue Limestone, dijjping 16° to 

 the N. E. and striking N. 15° to 20° W. East of this outcrop a bed of Blue 

 Limestone, as already mentioned, has been developed by a line of prospoct 

 holes along the face of White Ridge, whose elevation to its present relatively 

 higher position must necessarily have been the result of faulting. Data are 

 wanting, however, to locate definitely the line of this fault. That given on 

 the map as the Sherman fault is determined principally from the theoretical 

 considerations furnished by Section F, according to which it is assumed that 

 a certain arbitrary thickness of White Porphyry exists under the Blue Lime- 

 stone. The fault line would therefore have White Porphyry on either side 

 of it, which necessarily renders its position difficult to recognize. That such 

 a body does exist under the Blue Limestone is rendered almost certain l)v 

 the fact that it is found at this horizon farther westward, along the western 

 base of Mount Sheridan and throughout the Leadville region to the north- 

 east of a line roughly drawn fi-om Mount Sheridan to Fryer Hill. 



