148 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



tlie right or west a few steep lines, indicating the nearly vertical dip of the 

 beds adjoining the fault which is found at other points. x\ comparison of 

 the direction of the valley with tliat of the axis of the fold affords a ready 

 explanation of this deceptive appearance. The plane of the cliff section 

 stands at an angle of 60^ instead of at 1)0°, or at right angles with the 

 axis of the fold. So that nature has afforded a graphic illustration of the 

 simple problem in descriptive geometry, the diagonal intersection of a 

 cylindrical body by a plane. 



The interior of the arch is made up of Archean rocks, mostly gneiss 

 with white vein-like bodies of pegmatite running through it. Over these 

 stretch the entire lower Paleozoic series, with some interbedded porphyries, 

 the principal of which is the Sacramento Porphyry in the Lower Quartzite, 

 corresponding ap2)arently in horizon with that on the north side of the 

 gulch. Blue Limestone, more or less eroded, forms the crest of the hill. 

 On the east side the beds slope away with the angle of the hill at about 

 20°. On the west of the crest, towards the fault, the dip rapidly steepens 

 and becomes vertical before reaching the fault plane. The structure can 

 naturally be best seen on the cliff face. Hei*e as elsewhere the stratified 

 series seems much thinner in a vertical than when in a horizontal posi- 

 tion. On the north face the Blue Limestone comes into contact with the 

 fault instead of the Parting Quartzite, as on Pennsylvania Hill. The rock 

 is much shattered and there is considerable development of black chert. 

 Apparently some slight ore deposition has also taken place; but there is no 

 evidence that this is the result of the faulting action. On the crest of the 

 ridge, still east of the fault, are some shales and beds of impure anthracite, 

 cliaracteristic of the lower part of the Weber Grits formation. 



West of the Sacramento arch the ridge is level for a short distance, and 

 then rises in a regular slope to the Gemini Peaks, two little projections 

 crowning the ridge opposite the head of Sacramento amphitheater, on the 

 north, and of Iowa amphitheater, on the west. The regularity of the struct- 

 ure lines on the eastern flank or back of this ridge is extremely remark- 

 able and is partiallv shown in the sketch. The dip of the beds, which to 

 the west of the fanll are (.■niirei)- of the Weber Grits formation, is here 

 sreeper than in the adjoining aniphilheater, averaging from 25'^ to 35°, 



