126 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. * 



talliiie, is composed of very uniform minute grains of quartz and feldspar, 

 with mica in opaque dots, and intrudes in bays into the quartz grains, which 

 are quite free from inclusions. 



To the northeast of the lake a considerable body of quartz-mica por- 

 phyrite was observed; but its exact form, whether a large dike or an iso- 

 lated mass, could not be determined. It closely resembles the rock already 

 described from the knoll south of Democrat Mountain. It is extremely 

 fine-grained, but of very dark color, owing to the large amount of biotite, 

 and contains no hornblende. Many other eruptive dikes were observed on 

 the face of the cliffs, which time did not admit of studying carefully. Prom- 

 inent among these is a dike or sheet of dark porphyrite, cutting the ridge 

 which divides the upper part of the amphitheater into halves. 



MIDDLE-EASTERN DIVISION. 



This division includes the eastern slopes up to the' crest of the range, 

 from the line of lower Buckskin Valley south to that of Horseshoe or Four- 

 Mile Creek. The region is crossed diagonally by the line of the London 

 fault, which divides it into two parts in such a manner that there is a repe- 

 tition of the same series of sedimentary beds exposed in the- canon sections 

 on given transverse lines. 



Glacial erosion. Evidences of glacial erosion are abundant in the valleys 

 of all the streams flowing from the crest of the range, but the data afforded 

 by Buckskin and Mosquito gulches is so definite as to seem worthy of 

 special mention. As the map shows, the two valleys are nearly parallel 

 and similar in general form, in that their main course in the Archean rocks 

 is southeast, the glaciers which originally filled them having been fed by a 

 very broad ne've'-field, filling two or more less distinct basins at their head, 

 and that in their lower course, where they reach the upturned edges of the 

 Paleozoic strata at the line of their steepening dip, they bend sharply to 

 the east and cross these strata approximately at right angles to their strike. 

 Just above the bend a raised bench or shoulder is found on the south side 

 of either cafion, several hundred feet above its present bottom, which is evi- 

 dently a portion of a former valley bottom, and marks approximately the 

 level to which the valley was cut out by the glacier which once filled it. 



