330 MINING INDUSTET. 



incloses crystals of wliite sanidin feldspar and black mica, but apparently no 

 free quartz ; it lias a dark, glazed, weathered surface, and frequently a columnar 

 structure, forming small pentagonal and hexagonal prisms. The line of contact 

 of this rock with the granite, which has a north and south direction, is marked 

 by a depression in the crest of the ridge west of Prometheus, and a ravine on 

 either side, showing that the granite was decomposed, and hence more deeply 

 eroded on this line ; and the pearlitic texture would seem to be due to the 

 reciprocal action of the granite on the rhyolite body. The main rhyolite of 

 Prometheus is also very easily decomposed by atmospheric agents ; hence, 

 in the low saddle of the main ridge above Austin and in various depressions 

 of the eastern spurs, it has been eroded off, and the underlying granite laid 

 bare ; here the granite is found to be so thoroughly decomposed on the sur- 

 face that it crumbles into fine quartz sand at the touch, the feldspar having 

 been kaolinized, and the mica only leaving its traces in stains of iron oxide 

 through the mass. In several places on the eastern slopes, however, the 

 granite is found in large rounded blocks, in a comparatively undecomposed 

 state ; these may not have been entirely covered by the flow of rhyolite, or 

 their outer decomposed surfaces may have been entirely worn away by the 

 action of erosion, and the undecomposed kernel left as now found. 



South of the pass above Austin extends a flat-topped ridge, having a 

 gentle slope to the eastward, which is formed of a reddish-brown porphyritic 

 rhyolite ; its compact, homogeneous, feldspathic matrix incloses small crystals 

 of sanidin feldspar, with no free quartz as far as can be seen by a simple loupe; 

 in the valley to the east this rock is found, having a lighter drab-colored ma- 

 trix, inclosing, in addition to the feldspar, crystals of black mica. 



Still further south, on the divide which connects this ridge with the hills 

 north of Greneva, and in a line due south from Prometheus, occur similar 

 rocks, and in the same relative position as on that peak; they are, however, 

 more compact, and have not the spherulites which are found in those rocks. 

 Still another variety of texture is found on the eastern limits of the body ; as 

 these, however, are, in a great measure, covered by the debris of the Park 

 basin, they are not so clearly defined as the western limits. On the northern 

 slopes of the hills, north of Geneva and near the Overland Stage road, is found 

 a brick-red rhyolite, having an earthy homogeneous texture, probably the 



