MOSQUITO PEA.K TO MOUNT EVANS. 1^9 



The description of the geological character of the region west of the 

 fault will now be resumed in topographical order as it lias been carried on 

 hitherto, taking up alternately the canon sections and intermediate ridges in 

 regidar succession as one goes south. 



Main crest from Mosquito Peak to Mount Evans. On the main CrCSt of the range, 



at the head of the south half of the North Mosquito Amphitheater, the fault 

 line is well marked by a sudden change from limestone to a coarse red 

 granite, in the saddle or notch between Mosquito Peak and the peak next 

 north of it. The upper tunnel of the Little Corinne mine, on the north fece 

 of Mosquito Peak, is run near the top of the Blue Limestone; above this are 

 12 feet of White Porphyry, while the lower tunnel of the same mine is in 

 White Limestone. The shales and quartzites of the Weber Grits formation 

 form the summit of the peak, included in which is a thin bed of Sacra- 

 mento Porpliyrv. 



From Mosquito Peak southward to Mount Evans the main crest of 

 the range is a nearly straight ridge, steeply escarped on the west. At 

 the base of this escarpment runs the Mosquito fault, by whose displace- 

 ment the Archean schists have been thrust up into juxtaposition with the 

 beds of Weber Grits formation on the west. The beds of the lower Paleo- 

 zoic series can be traced along the summit of this wall, descending gradually 

 to the southward, until under Mount Evans, in the Evans amphitheater, they 

 are found at the base of the slope. In this extent there is a slight break 

 in their continuity, occasioned by a transverse fault in a little ravine just 

 south of the zigzags of the Mosquito grade. The thin sheet of White Por- 

 phyry lying above the Blue Limestone, which is observed at Mosquito 

 Peak, disappears before reaching Mosquito pass; but the sheet of Sacra- 

 mento Porphyry 10 feet thick, which occurs in the lower portion of the 

 Weber Grits, apparently at the summit of the shale division of this formation, 

 graduall}' thickens to the southward, and on the eastern wall of the Evans 

 amphitheater it suddenly widens out into a body five hundred to seven hun- 

 dred feet in thickness. Owing to the sharp contrast of the angular and 

 almost Gothic forms, into which this mass weathers, with the horizontal 

 lines of the bounding sedimentary beds, its outlines can be readily distin- 

 guished even from so great a distance as Leadville itself, and would be seen 



