SQUAW VALLEY REGION. G13 



and foi-ms the divide between the waters of the Humboldt and those of the 

 Owyhee Eiver. The view to the northward from the summit of Rose Mount- 

 ain, the most northern point visited by us, extends out over an almost 

 limitless plain, whose horizon is as level as that of the ocean. The rhyolite 

 which forms the summit of Rose Mountain shows the appropriateness of the 

 name which has been given to this rock, signifying "thin flowing", since it 

 is made up of sheets of lava only about an eighth of an inch in thickness. 

 It is a red, compact, felsitic groundmass, containing only crystals of quartz, 

 with an occasional development of sanidin-feldspar, the surface of each 

 layer being colored by a red earthy coating, containing a few flattened 

 crystals of feldspar. 



A considerable variety of rhyolites was found along the southern slopes 

 of Rose Mountain. Among the most noticeable is a rhyolitic breccia, made 

 up of pink and red angular fragments of earthy rhyolite, whose outline is 

 generally rectangular, and which vary from half an inch to an inch or more 

 in diameter. The banded structure of the rhyolite, from which these frag- 

 ments are derived, gives to them the appearance of a structure like that of 

 woody fibre, so that the rock has the appearance of a mosaic made of seg- 

 ments of wood. One variety of whitish, decomposed rhyolite is full of 

 druse-like cavities, filled with prismatic crystals of quartz. 



Along the lower foot-hills is a development of dark pearlitic rhyolite, not 

 unlike that of Mount Neva, but less rich in crystals. Quartz and sanidin are 

 its prominent crystalline ingredients, but the microscope discloses also some 

 green augite. In this rock are found fragmentary inclusions of a dark-, 

 green, rather finely crystalline diabase, which is remarkably rich in olivine, 

 and shows well-defined tabular crystals of plagioclase and pale-brown 

 augite. Under the microscope, a little globulitic base is detected and some 

 titanic iron. 



At Sunset Gap, in the eastern end of Squaw Valley, is a similar black 

 pearlitic rhyolite, interstratified in a white porphyritic rhyolite containing 

 small crystals of black hornblende and mica, with sanidin and a little quartz, 

 in a white, rather earthy groundmass. The dark pearlitic rock contains, 

 besides the sanidin, some little plagioclase and little green augite, but no 

 quartz, hornblende, or biotite. These rhyolites show a decided dip to the 



