BLACK EOCK MOUNTAINS. 795 



only occurrence found, not directly connected with the Tertiary beds of 

 the Truckee Miocene, - 



The rhyolites were principally examined in the neighborhood of Hardin 

 City, where the mountains are wider than at the southern point, and the 

 peculiar structure of the hills already noticed more prominent. These 

 rhyolites are generally of earthy and brecciated varieties, and poor in 

 crystalline secretions. Their groundmass is characterized microscopically 

 by a medium character between the indistinct and the microfelsitic, as is 

 shown in Vol. VI, Plate VII, fig. 1. 



At the base of the cliifs east of Hardin City are outcrops of white 

 loose-grained rhyolitic breccias, having a gravelly structure, and containing 

 small pebble-like fragments of darker- colored rhyolites in a white felsitic 

 groundmass. They have an appearance of bedding, and are exposed in a 

 thickness of several hundred feet, showing varieties of texture, from the 

 loose crumbly nature above described to a tolerably compact mass, having, 

 however, the same gravelly composition. Above this occurs the so-called 

 Snowstorm Ledge, a bed of decomposed basalt, from 40 to 60 feet in thick- 

 ness. In its extreme form, this basalt is a greenish-drab earthy mass, 

 rendered almost like a sponge by the quantity of amygdaloidal cavities 

 running through it, which are generally filled with a green earthy pow- 

 der. Where the decomposition has not proceeded so far, the basalt is of 

 a light greenish-gray color, showing no macroscopical crystalline ingredients, 

 its mass still full of smaller or larger rounded cavities, some of which are 

 filled with the green earth, others again lined with botryoidal concretions 

 of chalcedony, and in others still the botryoidal form is preserved, but a 

 thin coating of hydrous oxide of iron only remains. The top of the cliff 

 is made up of a fresh, black, lustrous basalt, with clean conchoidal fracture, 

 ringing under the hammer, and of fine, even-grained texture; this has also 

 irregular crevice-like cavities, lined with a botryoidal coating of chalcedony, 

 and in some cases containing a little calcite. On the slopes of the adjoining 

 hills and ravines are innumerable geodes, lined generally with quartz crystals, 

 and sometimes disclosing agates of great beauty. The green earth filling 

 the pores of the decomposed basalt, being tested chemically, was found to 



