160 GEOLOGY AND MINING INDDSTEY OF LEADVILLE. 



silica, a form of vein material which, as will be seen later, is common in 

 the Leadville mines and easily to be mistaken for genuine qnartzite. The 

 brecciation is doubtless due to the action of the porphyry at the time of its 

 intrusion. 



The Horseshoe. — Horseslioe Mountain, as is shown both on the map and 

 on the sketch, is covered by a thin shell of easterly-dipping beds of the 

 lower Paleozoic series, whose angle on the crest is about 10° and steepens 

 to an average of 20° on the eastern slopes. The irregularity of the out- 

 crops of the successive formations shown on the map represents the results 

 of erosion on this thin shell. 



The character of the outcrops in the Horseshoe itself is sufficiently 

 shown in the sketch. Its peculiar form is a result of glacial erosion, which 

 alone could have carved vertically across the inclined surfaces of hard sedi- 

 mentary strata. The main body of the encircling cliffs is composed of the 

 White Limestone and of the upper beds of the Lower Quartzite, which, ow- 

 ing to their peculiar weathering, received in the field the convenient name 

 of "sandy limestones." On their weathered surface they resemble in all 

 respects a sandstone, but a fracture of the mass shows the interior to have 

 the compact semi-crystalline structure of limestone. The beds of Blue 

 Limestone above these are more or less eroded off, while the pure quartz- 

 ites at the base of the series are in .places concealed under the talus 

 slope of debris. In the very bottom of the amphitheater are two or three 

 little shallow lakes or ponds of glacial origin, carved out of granite or tho 

 Lower Quartzite. Passing down the stream fi'om the glacial amphitheater, 

 one crosses successively an ascending series of outcrops which sweep round 

 in graceful curves up the bounding ridge to join the beds on the crest of 

 the range. 



Intersecting these outci'ops in a northeasterly direction, and in part 

 following the line of contact between the Blue and White Limestones, is 

 a small body of porphyrite; this and a similar outcrop in the Four-Mile 

 Amphitheater constitute the only instances observed of the occurrence of 

 this rock within the White Porphyry region. The rock is a grayish- brown, 

 homogeneous-looking, fine-grained mass, showing small glistening black 

 biotites and minute white feldspar crystals with round quartz grains. L'nder 



