EEGION SOUTH OF GEANITE MOUNTAIN. 699 



graphical habit to one or two other basaltic outbursts, notably to the occur- 

 rence at the east base of Buffalo Peak, West Humboldt Mountains, by the 

 absence of globulitic and glassy matter, and the consequent prevalence of 

 crystallized minerals. Its aspect is more granular and more like diabase 

 than most Western Nevada basalts, and, at the same time, is wanting in the 

 vitreous lustre which is derived from included glass. The feldspars are 

 all richly striated, the larger ones containing obscure inclusions ; olivine 

 occurs more or less decomposed. 



Cottonwood Canon, east of Table Mountain, exposes the southernmost 

 limit of the Triassic mass already described, which here ends in a contorted 

 and broken mass of marble. From there, for 12 miles to the south, the 

 range on the east side is occupied by an outburst of rhyolite, of which all 

 the loftier portions, and indeed down to within 300 or 400 feet of the plain, 

 were formed by a series of sub-aerial ejections, while below that level they 

 were poured out into and under water. These sub-lacustrine tufas are 

 excellently stratified, and show the usual decomposition and infinitesimal 

 crackings of the quartz-grains. It is doubtful if there is any place in the 

 whole region of the exploration where the rhyolitic tufas are more interest- 

 ingly displayed. 



Lying to the west of this rhyolite body, and forming the summit of the 

 range, is a north and south ridge of diorite, culminating in Chataya Peak. 

 It forms a broad compact mass, cut by numerous canons, and almost com- 

 pletely surrounded by the later volcanic rocks. These dioritic masses are 

 beautifully crystalline, very coarse-grained, and, like the diorites of the Sou 

 Spring group above described, contain a large amount of kaolinized feldspar, 

 in this case revealing, under the microscope, scarcely any twin striation. 

 The hornblende has so much parallelism of arrangement that the rock in 

 some instances has a gneissic structure. There is a little free quartz, and 

 the microscope reveals considera,ble apatite, with some titanic iron. In this 

 diorite are certain clouded passages, which have a fine-grained compact 

 arrangement, apparently of the same mineral constituents, in which, 

 however, some of the feldspars have remained pellucid, and show fine 

 striation. This rock possesses no biotite. Considerable chemical change 

 has gone on in the hornblende in some places. On the western foot-hills of 



