SHOSHONE EANGE. G23 



the transportation of metallic oxides through the rock simply by attraction 

 of heat. 



The rhyolite, which scores the east base of the range, in the region of 

 Shoshone Peak and to the southward, possesses a characteristic rhyolitic 

 gToundmass, with well-developed crystals of quartz and feldspar. In color, 

 it exhibits various shades of lilac, light brown, and purple. The rock is full 

 of seams, which give evidence of later solfataric action, and are stained with 

 a slight deposition of sulphur and sulphate of lime. The quartz-grains in 

 some localities are surrounded by a coating of maroon-colored ferruginous 

 and siliceous material, which gives to the broken fragments of quartz the 

 color of currant-jelly. In other localities, this coating is nearly black in 

 color. In places, the groundmass has a peculiar crushed sugary ap23earance, 

 and the small interstices are filled with a clear siliceous binding material, 

 probably chalcedonic in nature. 



In the large canon which leads up into the range from about 4 miles 

 north of Garico Lake are obscure outcrops, which appear over quite a 

 wide area of country, of earthy- white and cream-colored rhyolite, which 

 is composed of a homogeneous felsitic mass, containing no crystals, and 

 revealing under the microscope scarcely any quartz or biotite. From the 

 extreme fineness of the material and its earthy character, it would seem to 

 be a tufa, and was probably ejected under a fresh-water lake, which for- 

 merly filled Crescent and Carico Valleys. Nowhere else along the belt of 

 the Fortieth Parallel Survey has a tufa exactly similar been observed. It 

 bears a close resemblance to some of the sandy beds of the Truckee Mio- 

 cene, which are rich in infusorise, but under the microscope, with crossed 

 nicols, it is clearly seen to possess a crystalline groundmass. Some of the 

 rhyolitic tufas of the Truckee Miocene are of mixed origin, being composed 

 partly of ejected trachytic material and partly of lacustrine sands, and it 

 is not improbable that this Shoshone rhyolite also carries some fine detrital 

 matter. It is characteristic of some of the finest-grained rhj^olitic tufas 

 that they show no planes of stratification, and indeed this is usually the 

 case in the absence of much mica and hornblende. Where the tufas are 

 chiefly felsitic, under certain circumstances of deposition they reach a con- 

 siderable thickness, often not less than 50 or 80 feet, without showing any 



