704 DESCKIPTIVE GEOLOGY. 



gradually toward the west, reaches on the opposite side of the valley an 

 elevation but a few hundred feet above the Quaternary gravels. It is of 

 considerable interest, as it forms a divide across the valley, completely 

 shutting in the Carson and Humboldt Desert to the south, and is one of the 

 few prominent cross-ridges of Tertiary eruptive masses connecting the 

 longitudinal ranges of the basin. Lithologically these basalts agree with 

 those described from the Sou Spring Hills, a deep black, crystalline mass, 

 with great uniformity of texture. 



From Sommer's Pass to the southern end of the map, the Pah-Ute 

 Range consists essentially of a granite mass overlaid by sedimentary strata, 

 which have a prevailing dip to the east. It has been less studied than any 

 other portion of the range, but from the occurrence of porphyroidal rocks 

 of great thickness, resembling the beds of the West Humboldt Range, and 

 underlying dark limestones and marls, it has been provisionally referred to 

 the Triassic. 



South of the line of the map, basalt again comes in, covering a wide 

 field and obscuring all the older rocks. It differs from the other basalts 

 in having a characteristic glass base, which is indicated by its peculiar 

 fracture under the blow of a hammer, and readily seen in thin sections 

 under the microscope. It carries a few well-developed sanidins and some 

 olivine. The glass base is much altered, as shown in Vol. VI, Plate XI, 

 fig. 1, which represents a thin section of a specimen obtained from Mount- 

 ain Wells Station, on the old Overland Stage Road, a few miles south of the 

 limit of the map. Before returning to the region north of Granite Mountain, 

 the thermal springs which lie at the eastern base of the range and the 

 immense salt-fields of Osobb Valley require some special mention ; the for- 

 mer are of interest from their evident association with the volcanic activity 

 of the range, and the latter from the economic questions connected with 

 the' deposits. 



Sou Hot Springs. — Directly south of the rhyolite hills, which extend 

 from McKinney's Pass, and about one-quarter of a mile from the foot of 

 the slope, there rises a low mound of hot-spring tufa, covering perhaps 12 

 acres of ground. It has built itself up to a height of at least 60 feet above 

 the plain, and has generally the figure of a broad, low dome rising out of the 



