TERTIARY RHYOLITES OF NORTH AMERICA. 273 



associated with quartz. Like the hornblende, it is usually associated 

 with magnetite. Apatite and magnetite are diffused in many rhyolites, 

 but both are sometimes absent. There are few, if any, accessory 

 minerals in rhyolite, and when found they are always integral parts 

 of the ground mass. 



North American Rhy elites. Rhyolite is the predominant super- 

 ficial volcanic rock of the Fortieth Parallel, where its oldest eruptions 

 appear to date from the beginning of the Pliocene period. It accom- 

 panies the trachyte in the Rocky Mountains, but there are no important 

 exposures between the Rocky Mountains and the western side of the 

 great Salt Lake Desert. Between the ii4th meridian and the borders 

 of California, rhyolites are widely spread. Many of the exposures 

 occur at the angles of great flexures of the rocks. Against the base 

 of Mount Richthofen rhyolites are poured out in immense streams. 

 Here their colour is dark, and the ground mass is a fine-grained 

 mixture of broken crystals of sanidine and dark quartz, in which are 

 contained large clear grains of quartz, shining black hornblende, and 

 large fractured sanidine ; but besides the sanidine, in some localities 

 orthoclase crystals are found, but then the rock has a more felsitic 

 character, and the ground mass is usually light. The rhyolite of 

 Desert Buttes is sometimes warm grey, sometimes salmon colour, and 

 contains spherolites and lithophyses, and is reticulated by veins of 

 translucent chalcedony. At times the rhyolites are glasses, variously 

 banded and tinted, containing sanidine and quartz. 



In the region of the Washoe Mountains, to the south-west of Salt 

 Lake Desert, rhyolites are grandly spread, presenting excellent ex- 

 amples of viscous flow, and of variations in rock structure, sometimes 

 being green and compact, like hornstone, at other times brick-red and 

 porphyritic, with sanidine and prisms of hornblende. 



North of Spring Canon the rhyolites include pumice, and bril- 

 liantly tinted glassy and half-glassy rocks, the quartz in which has 

 inclusions with moving bubbles. At Clover Peak the rock is as black 

 as basalt, and free quartz has a brilliant olive-green tint. The tuffs 

 of this region are sometimes creamy or light-grey in colour, sometimes 

 pale-green or olive-tinted. The rhyolites of the Sierra Nevada are 

 among the most important in the world. They form the Augusta, 

 Fish Creek, Shoshone, Toyabe, Cortes, Seetoya, and part of the 

 Pinon ranges, and the Mallard Hills. This belt was explored for a 

 length of 200 miles and a breadth of 40 to 80 miles. 



North of the Humboldt River at Osino Canon, the rock is rich in 

 crystalline constituents, and contains sanidine, biotite, and quartz. 

 Biotite is also met with in the ground mass of rhyolite in other locali- 

 ties, but is usually rare. In Pinon Pass there is more black mica 

 than in any other exposure. In the Shoshone Mesa the rock presents 

 several varieties besides the ordinary type, with sanidine, cracked 

 quartz, a little plagioclase, and occasional mica. One of these is 

 pearl grey, rich in tridymite, and poor in crystalline secretions. Others 

 are dark pearlites, with more or less augite, and characterised by 

 spherolitic concretions. In several localities, as at Cortes Peak, 

 VOL. I. S 



