666 DESGKIPTIVE GEOLOGY. 



From its mode of occurrence in direct contact with the acidic rhj^olites, 

 this basalt merits special mention. In its superficial physical aspect, it resem- 

 bles an ordinary scoriaceous basaltic cinder, with a very cellular sponge-like 

 structure and grayish-black color. When struck with a hammer, it emits 

 a clear ringing sound, and fractures like a rock rich in a glassy or half- 

 glassy base. The most striking lithological feature of the rock is the 

 occurrence of large white vitreous feldspars an inch or more in length 

 imbedded in the dark base. No other mineral constituents have been 

 recognized by the unaided eye. Professor Zirkel has shown, under the 

 microscope, in polarized light, that the large feldspar crystals are wanting in 

 the characteristic striae, and suggests that they may be sanidin. But, on the 

 other hand, all the smaller microscopical feldspars are without doubt plagio- 

 clase, the base being made up of triclinic forms associated with augite. 

 No olivine or other minerals can be detected, even under the microscope. 

 The occurrence of such large, fresh sanidin crystals in a basaltic lava is in 

 itself somewhat exceptional, but the interest is heightened by finding them 

 in a rock which has poured through a rhyolite cone rich with secretions 

 of the same feldspar. It seems highly probable that they may have been 

 derived from the older rock, being caught up by the liquid lava and held 

 intact as an accessory constituent, a view which finds support in their mode 

 of occurrence. 



There should be mentioned in connection with this volcanic region two 

 localities of warm springs, situated in the valley to the westward of the 

 Fish Creek Mountains, which come to the surface on the border of a broad 

 Lower Quaternary plain. The water is clear, and, when cold, palatable; 

 the temperatures were not ascertained. 



Battle Mountains. — The Battle Mountains lie on the west side of 

 Reese River Valley, directly west of the 117th meridian, and opposite the 

 highest portion of the Shoshone Range. They occupy but a small area, and, 

 although Sue Peak and Antler Peak rise nearly 4,000 feet above the Hum- 

 boldt Valley, the group is not more than 16 miles in length by scarcely 12 

 in width, and both geographically and geologically stands in a somewhat iso- 

 lated position. On all sides, the group is surrounded by broad, open Quater- 

 nary plains, which northward stretch far beyond the limits of the map. 



