IS^ORTHERN COKTEZ EA^'GE 611 



pact, heavy rock, having something of a columnar structure, of a dark 

 reddish color on its weathered surfaces. Macroscopically, it shows 

 only small plagiocla.se and black hornblende crystals in a dark-bluish, 

 or sometimes greenish-gray, felsitic groundmass. Under the micro- 

 scope, the hornblende has rather a greenish than the cliaracteristic dark- 

 brown color of andesite, and is somewhat fibrous. The groundmass is a 

 yellowish-brown mass abounding in black trichites, containing some apatite, 

 but neither augite, biotite, nor tridymite. These three rocks, which are so 

 closely connected geologically, have, in their mineralogical composition, 

 therefore, a similar interchange of characteristics. 



The rhyolites which have covered these older volcanic rocks along the 

 western borders of Independence Valley are generally white, decomposed 

 varieties, which are almost amorphous, the only traces of crystals being 

 white outlines of kaolinized feldspar. By the aid of the microscope, they 

 are seen to have somswhat of a trachytic appearance, the groundmass 

 showing no tendency to fibration or fluidal structure, and the presence of 

 some altered hornblende and biotite, together with a good deal of quai'tz, is 

 detected. On the extreme foot-hills of the range, about four miles north of 

 Tuscarora, is a bed of dark rhyolite, in some respects almost like an andesite. 

 It contains, however, almost exclusively sanidin -feldspar in a dark, compact, 

 felsitic groundmass, with mica in remarkablywell-defined hexagonal prisms, 

 and a little free quartz. 



In the ravine a few miles south of Tuscarora, a body of augite- 

 andesite is exposed under the rhyolite flows. It has a dark-gray ground- 

 mass, with somewhat of a resinous lustre, full of crystals of plagioclase 

 and augite. The latter are remarkably well defined, while the plagioclases 

 are seen to be full of fragments of yellowish glass. Under the microscope, 

 the groundmass is seen to be made up of microlites in a glassy base of gray 

 color, and to contain no olivine. The rh3'olite adjoining this body of augite- 

 andesite, and forming the foot-liills along the southern portion of Independ- 

 ence Valley, is a white porphyritic variet}^, containing a considerable 

 development of mica, quartz, and sanidin in a somewhat decomposed 

 felsitic groundmass. 



The white amorphous rhyolites extend up on the eastern slopes of 



