DESATOYA MOUNTAINS. 647 



green color, containing some pumice-like fragments, which themselves are 

 of a darker green, approaching the color of the diorite. Associated with 

 this is a rhj^olite which contains breccia-like fragments of the adjoining 

 quartzite and large crystals of quartz and mica, with well-preserved sanidiu 

 feldspars in a yellowish-gray, hornstone-like groundmass, which, under the 

 microscope, is seen to contain microscopical biotite and apatite, and large 

 sphserulites with a distinct centre, between which are axially fibrous bands 

 of felsitic material. 



At the head of the South Canon, to the west of the mines, is a red, 

 somewhat earthy rhyolite, also containing ijicluded fragments of a greenish 

 decomposed rhyolite rich in crystals of sanidin and quartz. The ground- 

 mass contains large dark-yellow sphserulites, which in section show a con- 

 centric structure like the cross-section of a tree. On the western foot-hills 

 of the range, adjoining the limestones, is a dark-red porphyritic rhyolite 

 very rich in apatite, which, in contact with the limestones, is decomposed 

 into an earthy variety, through which run seams of calc-spar and lenticular- 

 shaped aggregations of calcite crystals, which are evidently derived from 

 the limestones. In the rhyolites of the eastern foot-hills are found in 

 the quartz remarkably large glass-inclusions, having a peculiarly-shaped 

 bubble, and included portions of the groundmass, which have assumed a 

 hexagonal shape. The form of the bubble is shown in Vol. VI, Plate I, 

 fig. 16. 



To the south of the New Pass Mines, the Desatoya Mountains are rep- 

 resented by a narrow ridge having an elevation of only 1,500 to 2,000 feet 

 above the valleys, made up of flows of different rhyolites. A section of 

 the range is obtained in the New Pass, a canon-like gorge, which cuts entirely 

 through it, at right angles to the trend, to a depth of about 1,000 feet below 

 the summit. A succession of rhvolites is found here similar to that seen on 

 the Mount Airy Hills. In the centre of the ridge is a great thickness of earthy 

 breccias, of colors pinkish-red above, white and green below. The thickness 

 of these bodies cannot be less than 600 feet. The greenish breccia-tufa is 

 a mass of a pale-green color, containing pumiceous fragments of a more 

 intense green, enclosing frequently crystals of quartz and feldspar. A mi- 

 croscopical examination of this curious breccia-like tufa shows that it is 



