EEGION SOUTH OF GEANITE MOUNTAIJT. (J97 



duce, in the habit and mode of weathering " of these hthoidal varieties, 

 many sharp contrasts to the more crystalhne and vesicular ones, inas- 

 much as they disintegrate less rapidly, develop scarcely any large feld- 

 spars or quartz-grains, and break with a smooth cherty fracture, affording 

 steep slopes and abrupt walls. In the canon which trends north from the 

 Sou Hot Springs is a remarkable dark indian-red variety, consisting almost 

 exclusively of a fine lithoidal base, in which are a few sharp, brilliantly 

 defined, and entirely fresh crystals of sanidin and minute particles of quartz. 

 Through this base are waving ribbon-like bands and strings of minute 

 fibrous material, also more or less distinct aggregations of sphserulites, and 

 narrow lines of devitrified glass. The latest of the flows, capping the others, 

 contain well-developed sanidin, a few small biotites, and concentric radial 

 sphserulites. The flows of middle age appear to be chiefly lithoidal, while 

 the earliest of all are formed of brecciated material. Here, as in many 

 other localities among the rhyolites, the included fragments are composed of 

 the same material as the binding paste ; the latter, however, is more finely 

 felsitic, the crystallized minerals being very minute ; whereas, in the included 

 fragments, there are large dihexahedral quartz crystals, and sanidins one- 

 eighth of an inch in length. A peculiar feature of this breccia is that the 

 forms of the included fragments are rounded, and show, in the outer one- 

 eighth of an inch of their section, decided caustic phenomena. In some 

 instances, where the included fragments have been considerably fissured, and 

 earthy decomposition has taken place, the sphserulites are destroyed, leaving 

 spherical cavities ; the whole mass being tinged reddish-yellow by the infil- 

 tration of iron oxides, which are probably developed from the ferritic needles 

 in the groundmass. Here, also, it is noticeable, as in many other localities, 

 that the breccia-flows form the earliest of the rhyolitic series. These lith- 

 oidal varieties of rhyolite differ so characteristically in their physical aspect 

 from those found on the opposite side of McKinney's Pass, an analysis of 

 which has already been given, that an analysis of the indian-red rock just 

 described was made by Mr. R. W. Woodward, to determine, if possible, any 

 marked distinctions of chemical or mineralogical composition. The two 

 analyses agree very closely, showing less variation than may be found in 

 any two highly crystalline rocks of the same species. In this rock, from 



