836 DESCRIPTIVE GEOLOGY. 



these trachytes hold in time the period of the basalts, they show also an 

 affinity with that group by the presence of augite. In a geological sense, 

 though not petrographically, they may be considered as basalts with the 

 olivine left out, and sanidin substituted for a portion of the plagioclase. 

 Besides the dark trachyte-dikes above described, there is also another set, 

 having a north and south strike and a nearly vertical position, but with 

 very limited overflows. They consist of black vesicular basalts, and are 

 characterized by frequent coatings of hyalite, varying from one-quarter of 

 an inch in thickness down to a mere siliceous glaze. 



On the river-mesa, near the east end of the Truckee Canon, and east of 

 the diorite outcrop, are four isolated hills of normal sanidin-trachyte, having 

 a great similarity to the brown variety on the summit of Sheep Corral 

 Canon. 



The large fields of basalt which he to the south of the river and cap 

 the higher summits have never been entered or examined by our parties, 

 and it is quite probable that in the deeply eroded canons may be found 

 many outcrops of the earlier volcanic rocks. At the base of the range, 

 just south of Wadsworth, partially buried by drifting sands and tufas, is a 

 group of low hills, which cannot at first sight be distinguished from genuine 

 basalts, yet which even in the field present certain peculiarities of form and 

 texture that distinguish them from the surrounding rooks. They, are compact 

 fine-grained rocks of a resinous lustre, and varying from black to grayish- 

 black in color. Except minute feldspars, scarcely any mineral constituents 

 can be determined by the naked eye. The value of methods brought to 

 bear by the microscope in the analysis of rocks is well shown in the study 

 of this group. Professor Zirkel' regards them as augite-andesites, and has 

 given a detailed description of their structure and composition. Mineralog- 

 ically they appear to hold both sanidin and plagioclase in equal propor- 

 tions, but the minute crystals are mainly triclinic. Augite, however, greatly 

 predominates over hornblende. A determination of silica in one of these 

 rocks gave 59.99 per cent., an amount much too high for normal basalt, 

 but within the limits admitted for augite-andesites. Geologically, their true 



^ Microscopical Petrography, vol. vi, 224. 



