LONDON FAULT. 137 



at the southeastern corner of the map. This is the London fault, so called 

 from the hill dividing the two heads of the Mosquito gulch, through which 

 it passes. Its effects can be readily traced by the traveler who approaches 

 Leadville over the Mosquito pass. The Mosquito pass road, following up 

 the valley bottom of the north fork of Mosquito gulch, winds up the steep west 

 wall of the gulch and, passing through the narrow notch between London 

 Hill and the main crest of the range, ascends gradually in a southwest direc- 

 tion to the Mosquito pass. Up to the point where it reaches the northwest 

 wall of London Hill the rocks around are of gneiss and granite ; from there 

 to the summit of the pass is a confused mass of huge loose blocks of coarse 

 quartzitic sandstone and fine-grained porphyry, in which it requires a trained 

 eye to distinguish any definite structure lines, although the change in the 

 character of the rocks is evident to all. Looking south from the road across 

 the broad basin of the South Mosquito amphitheater, the eye is at once 

 attracted by the peculiar appearance of the ridge which forms its south wall. 

 This is the summit of Pennsylvania Hill. As seen from this distance, paral- 

 lel with the regular and comparatively gentle slope of its surface eastward 

 there can be distinguished along its upper wall a few horizontal lines mark- 

 ing the bedding planes of the Paleozoic strata, below which the steep face 

 of the hill shows no definite structure lines on its rocky surface, save those 

 which mark the talus slopes of broken rock accumulated towards its base. 

 The smooth, regular slope is broken at its crest by a dark knob, around 

 which the rocks are greatly discolored, and the debris from which presents 

 brilliant hues of yellow and red. West of the knob the outline of the hill 

 presents terrace-like escarpments, descending nearly to the level of the 

 amphitheater. The face of this portion of the hill shows regular stratifica- 

 tion lines, dipping eastward at an angle of 20, which can be seen with 

 great distinctness to its very base, where they are concealed by the talus 

 slopes. All end abruptly to the east before reaching the discolored knob. 

 This break in the continuity of the stratification lines, or of the beds which 

 they outline, is evident at the first glance, as marking the line of a great 

 fault plane. The evidence of the existence of this fault can be seen with 

 equal distinctness on the walls of the canon gulches to the south of Mos- 

 quito gulch, although in either case the conditions vary, both in the hori- 



