180 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



places undoubted g-lass substance is present. The rock also contains frag- 

 ments of another eruptive rock, in some respects resembling the Gray Por- 

 phyry, and in whose cryptocrystalline groundmass are numerous aggre- 

 gates of tridymite. 



From the head of Union gulch northward, on the west of the svncline, 

 the outcrops of Lower Quartzite and White Limestone grow wider, owing 

 to shallowing dip, till thej' are cut off by the valley of Empire gulcli, 

 and are succeeded on the west by the underlying granite. On either side 

 of the syncline the Blue Limestone forms j^rominent outcrops or ridges. 

 On the north end of the syncline, toward the ravine which runs down to 

 a little lake adjoining the meadow of Empire gulch, is a small body of 

 Gray Porphyry, apparently occurring between the Blue and the White 

 Limestone. Following the line of the fault northward from the head of 

 Union gulch, the Lower Quartzite, White Limestone, and Blue Limestone 

 are found successively in contact with the granite; and tinally the White 

 Porphyry almost touches it. Farther north the series is reversed, until in 

 the bed of the ravine at the foot of the north end of Empire Hill granite 

 is exposed on either side of the fault. There is here an anticlinal fold 

 whose axis corresponds with the major strike and from whose crest the sed- 

 imentary series have been removed down to the Lower Quartzite. Con- 

 tinued north, the line of the axis of this anticline nearly corresponds with 

 the Mike fault, which is first seen on the north wall of Empire gulch and 

 which will be described in detail in the chapter devoted to the vicinit}' of 

 Leadville.^ 



Union fault, which thus far has followed the foot of the steep slope of 

 Empire Hill, now cuts across the northwest spur of this hill, and beyond 

 Empire gulch, after crossing Long and Deny Hill, joins Weston fault. The 

 displacement of this fault, like that of most of the faults of the region, is an 

 upthrow to the east. Consequently in ascending the steep northwest spur 

 of Empire Hill from the meadows in Empire gulch or from the anticline 

 above mentioned, one crosses a double series of easterly-dipping lower 



'Hy an error of the engraver, overlooked in proof-reading, the line of Mike fault has been carried 

 across Empire gulch to a connection with Union fault, following what was intended to be simply the 

 dividing line between the Cambrian and Silurian formations. 



