140 GEOLOGY AND MINING INDUSTRY OF LBADVILLB. 



in the heliotype view on page 6 had the photographic picture been equally 

 distinct with that which is formed on the eye of the observer. At the point 

 where the Mosquito grade descends this steep western wall the Lower Quartz- 

 ite comes in contact with Weber Grits on the west of the fault, but to the 

 north and south of this point Archean exposures intervene between the base 

 of the Paleozoic and the line of the Mosquito fault 



On the crest of the ridge are two irregular-shaped bodies of Sacramento 

 Porphyry at a higher horizon than the sheet already mentioned, which are 

 supposed to be the relics of a second intrusive sheet. The first of these 

 forms the summit of the peak next south of the Mosquito Peak, and can be 

 traced down its eastern slope across the Mosquito grade. The second forms 

 the crest of the ridge for some little distance south of Mosquito pass. In 

 the saddle west of London Hill the road crosses another exposure of por- 

 phyry, which is supposed to be the outcrop of the lower sheet of Sacra- 

 mento Porphyry exposed along the western face of the crest; while in the 

 sharp, prow-like point of London Hill is another interbedded sheet of Sacra- 

 mento Porphyry, which, as indicated in Section C, is presumed to be a con- 

 tinuation of the upper body, which is found on the crest of the range. 



South Mosquito amphitheater. The bed of this basin is formed by coarse 

 sandstones and grits of the Weber formation, dipping 20 to the east with 

 its slope. On the exposed faces of these beds glacial grooves and striae 

 are often very distinct. In the sandstones are various beds of porphyry, 

 and among the debris piles of huge rock fragments split off by ice and 

 frost, which form the steep slopes of the eastern and southern walls, por- 

 phyry forms an important element. Time did not admit of tracing out 

 the outlines and relations of all these porphyry bodies, and the structure 

 given on the map and sections, which assumes that the lower sheet which out- 

 crops along the west side of the crest of the range once extended over the 

 whole basin, may be only partially correct. 



Sacramento amphitheater. Big Sacramento gulch, like those to the north of 

 it, was once occupied by a glacier, and the amphitheater at its head, like 

 that of the South Mosquito, has been probably but little deepened since 

 Glacial time. The deeper cutting of the present stream extends some little 

 distance above the fault; below the fault line the bottom opens out into 



