150 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



actual contact is, however, obscured by broken masses which almost invari- 

 ably cover the- surface in these high regions. On the east side of the east- 

 ern of the Gemini Peaks, however, were foi.nd a few beds of Weber Grits, 

 within which was a small body of White Porphyry, while at either side of 

 the Weber Grits was found Sacramento Porphyry. It seems, therefore, that 

 this fragment of Weber Grits, with the included White Porphyry, was caught 

 up within the later outflow of Sacramento Porphyry. Such caught-up 

 masses of sedimentary rocks entirely included in porphyry masses are by 

 no means uncommon. 



On the southwest face of the western of the Gemini Peaks are beds of 

 Weber Shales, about fifty feet in thickness, consisting of gray limestone, 

 quartzite, and green micaceous shales. About half a mile south of this, and 

 in a shallow depression between the summit of Mount Sherman and the out- 

 lying shoulder to the east, is a similar succession of beds, dipping however 

 to .the west, which are entirely included in the surrounding mass of White 

 Porphyry. On the east of this shoulder again, at the contact of Sacramento 

 Porphyry and White Porphyry, are found thin beds of white quartzite, 

 belonging undoubtedly to the same general horizon. 



The most characteristic exposures of this great mass of Sacramento 

 Porphyry can be seen at the heads of Little and Big Sacramento gulches 

 and on the main ridge between Sacramento and Evans amphitheaters. On 

 the eastern wall of the latter it covers the greater part of its steep surface, 

 widening and rising to the southward, and sleeping up to the summit of Dyer 

 Mountain, where a thickness of some four hundred feet still remains. Below 

 this, and separating it from the Blue Limestone, is a remnant of the lower 

 beds of the Weber Grits formation, a relic of which forms the summit of 

 West Dyer Mountain. From the saddle between Dyer Mountain and Gemini 

 Peaks both Weber Grits and Sacramento Porphyry have been removed, 

 leaving the crest of the ridge composed of White Porphyry. The limits of 

 the two bodies of White Porphyry and Sacramento Porphyry are well 

 defined by a line running nearly northwest and southeast between Gemini 

 Peaks and Dyer Mountain. To the northeast of this line the White Por- 

 phyry rapidly thins out and disappears. The occurrences of this rock, hith- 

 erto noted in the regions farther north, were generally in the form of dikes 



