566 DESGRIPTIVE GEOLOGY. 



of the Pinon Range, and southward with the great dome-shaped masses 

 stretching from Robert's Peak to the Pinon Range, forming a north and 

 south line of eruption nearly 25 miles in length. 



Rhyolites skirt the northern foot-hills, connecting with those of the 

 Wah-weah Range, but the most interesting occurrence is foimd near the 

 summit, just north of the main peak, where it breaks out through the lime- 

 stone in a rounded dome-shaped hill. Rocks of precisely similar character 

 to this were nowhere observed throughout the Great Basin, and, although of 

 volcanic origin, it presents, in the field, a somewhat slaty schistose structure 

 rarely observed in rhyolites. The groundmass appears quite uniform in 

 texture, and of a dark bluish-gray color. Large feldspars are rare. Well- 

 developed hexagonal plates of biotite are abundant, associated with occa- 

 sional dark hornblende crystals. But for the large amount of finely-dis- 

 seminated quartz in the groundmass, the rock, like the rhyolite from the 

 east side of the Pinon Range, would show a close relationship with the tra- 

 chytes. In this connection, therefore, it is of special interest to know that 

 the microscopical analysis shows the same trachytic habit, but with many 

 marked characteristics of rhyolite. Apatite may be detected in thin sec- 

 tions under the microscope. 



Wah-weah Range. — This range extends from Garden Gate south- 

 ward for nearly 30 miles, and forms the western border of Keyes Valley. 

 It separates Keyes from Cor.tez Valley, the latter lying between 400 and 

 500 feet below the level of the former. Only about 8 miles of the extreme 

 northern end of the range is represented within the limits of the map, which 

 consists mainly of granite and quartzite. 



Granite occurs a few miles below Garden Gate, on the western side of 

 the range, where it rises abruptly above the long slopes of Quaternary 

 deposits of Cortez Valley, and forms a broken irregular-shaped mass for 

 some distance, reaching an altitude nearly 1,000 feet above the plain. But 

 little opportunity was afforded for studying this granite ; it is, however, a 

 medium-grained rock, resembling closely in its general outline the Mount 

 Tenabo and Shoshone Wells body, and is probably simply an extension to 

 the southward, Avith Garden Gate occupying a depression in the mass, 

 which rises on both sides to a considerable elevation. Surrounding the 



