136 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



and to this change of strike the synclinal structure observed may be in part 

 due. 



Along the northern wall, west of the Mosquito section just described, 

 the Cambrian and Silurian strata form a thin capping to the Archean cliffs. 

 At the western point of the hill decomposed porphyry is still traceable in 

 the Cambrianj but at the very highest point, near the line of the London 

 fault, only White Limestone is found at the surfsxce. This can be seen to 

 bend over in an anticlinal fold before it is cut oflf by the fault, and a promi- 

 nent quartzite crag, which will be described later, is assumed to be a portion 

 of the Parting Quartzite which has escaped erosion, standing in a vertical 

 position on the west side of this anticline and adjoining the fault plane. 



On the south face of the cliff overlooking Big Sacramento gulch an 

 eminence of the ridge just east of the fault line is capped by a body of 

 Sacramento Porphyry about one liundred feet in thickness. Over this, on 

 the eastern flank of the ridge, whose slope is but little steeper than that of 

 the strata, are further beds of white quartzite, succeeded lower down by 

 the overlying White Limestone. This white quartzite therefore represents 

 the Cambrian formation, and the Sacramento Porphyry an interbedded sheet, 

 here locally developed in unusual thickness. At the foot of the steeper 

 eastern slope of the ridge is found the Blue Limestone, over which is a bed 

 of decomposed Sacramento Porphyry, almost identical with that which is 

 characteristically developed in the Sacramento mine on the same hon'zon. 

 Zones of decomposition are characteristically marked on this rock by con- 

 centric lines, stained red by oxide of iron, the very kernel of the larger 

 blocks sometimes, though rarely, showing the original bluish color of the 

 unaltered porphyry. 



London fault. — The rcgiou tlius far described has been one comparatively 

 free from faults, the movements of displacement being, as it were, within the 

 beds, and generally not more than one hundred feet in amount ; move- 

 ments which have exerted no perceptible influence on the character of the 

 topography and have made comparatively little change in the geological 

 outlines. 



South of Mosquito gulch the eastern slopes of the range are divided 

 by one great fault line running diagonally across them, and finally dying out 



«i 



