MOUXT AIEY HILLS. 643 



rude moulds of shells, which have been referred to the Truckee Miocene, 

 though they may possibly be of later formation. The upper bed is a thin 

 seam of curious green compact sandstone, made up largely of fragments of 

 crystals of quartz, feldspar, and mica, enclosing angular particles of quartz- 

 ite and jasper. 



Mount Airy Hills. — The low hills between Reese River Valley at 

 Jacobsville and Smith's Valley, in the neighborhood of Mount Airy stage- 

 station, present an interesting succession of rhyolitic beds from 50 to 200 

 or 300 feet in thickness, Avhich have a general dip to the northeast, present- 

 ing bluff faces to the southwest. The lower beds are generally of a light 

 volcanic tufa or trass, varying in color from white to a delicate mauve, 

 made up of fragments of a pumice-like material, having a fibrous, woody 

 structure, and particles of black volcanic glass enclosed in a porous ash, 

 which carries occasional crystals of sanidin. Associated with this are beds 

 of a hard, rhyolitic breccia, made up of angular fragments of green and 

 gray rhyolite, and crystals of quartz which are often broken, together with 

 feldspar and biotite. The rock is frequently quite porous, and the cavities 

 lined with a coating of fine, colorless crystals, which, under the microscope, 

 are seen to present the form of a rectangular prism, with vertical striation 

 on the prismatic faces, which Professor ZirkeP thinks may be zeolites, either 

 comptonite or thompsonite. 



The rhyolitic lavas of these hills are reddish rocks, having an even, 

 conchoidal fracture, whose groundmass has somewhat of an earthy texture, 

 which are rich in crystals of quartz and sanidin, and contain little or no 

 mica or hornblende. The quartz is frequently smoky, sometimes so dark- 

 colored as to look like black glass. Under the microscope, the groundmass 

 is seen to have a fluidal structure, presenting stripes of different material, 

 some of which are axially fibrous, but with no tendency to sphserulitic 

 forms, and the quartz and sanidin crystals have no foreign inclusions 

 except glass. 



The low hills at the" head of Smith's Valley, which connect the Shoshone 

 Range with the Desatoya Mountains, are composed of various flows of 

 rhyolite, which have a general dip to the westward, away from the Shoshone 



^ Microscopical Petrography, vol. vi, 269. 



