THE KOCKS OF THE WASHOE DISTRICT. 71 



olivine, augite, labradorite, and magnetite, showing no glass excepting as 

 inclusions in the augites. The olivine occurs in part as fairly well developed 

 crystals, with hexagonal and octagonal sections, and occasional perceptible 

 cleavages. It is almost colorless, but shows the faintest possible tinge of 

 yellow. The decomposition amounts only to a slight discoloration along 

 some of the edges and cracks. Augite is present, in part in crystals as large 

 as the olivine, and in part in minute grains forming a portion of the ground- 

 mass. The feldspar is crystallized for the most part in lath-like forms, and 

 is often twinned according to the Carlsbad law, but in one or two cases 

 both albitic and periclinic twinning are visible. The determinable crystals 

 seem all to belong to labradorite. The magnetite is in no way remarkable. 



Field character. — The larger part of the basalt occurs in the form of ridges 

 with horizontal summits, giving the impression of tables, though they are 

 reall}^ ver}^ narrow. At the base of these ridges are numerous bowlders 

 which, under the action of frost, have assumed rounded forms. Besides the 

 areas visible on the map, there is a single bluff-like cropping near McClellan 

 Peak, where the bowlders have assumed an almost perfectly spherical shape. 

 It is hard, and rings like cast iron under the hammer, but is rather brittle 

 and chips readily. There is no considerable quantity of decomposed basalt 

 to be seen. 



This rock cannot be confounded with any other in the district, for it 

 all carries visible olivine, a mineral not met with in any other Washoe rock. 

 The elevation laid down as Basalt Hill is augite-andesite, and the rock 

 described by Professor Zirkel as an unusual basalt^ is both macroscopically 

 and microscopically the same as that here considered as metamorphic diorite. 



' Expl. of the 40th Par., Vol. VI., slide 528. 



