852 DESCRIPTIVE GEOLOGY. 



formation, and that the latter beds are composed simply of granitic detritus 

 from the surrounding ridges. To the eastward, this granitic group is topo- 

 graphically closely connected with, but geologically quite distinct from, the 

 Virginia Range, the volcanic outbursts of the latter resting as it were upon 

 the flanks of the granitic body, although rising to a much higher elevation. 

 In their physical attitude, the granite hills present but little that is of special 

 interest, the ridges being long and narrow, of monotonous forms, and 

 showing great uniformity of outline. State Line Peak is, however, an 

 exception to this rule, rising grandly above the surrounding country, nearly 

 4j00() feet above Honey Lake Valley. 



This granite may in general be characterized as a compact, dense, 

 medium-grained rock of a prevailing dark-gray color. So far as observed, 

 the rock-masses show no law in the arrangement of the mineral constitu- 

 ents, which usually present a fresh undecomposed appearance. Both 

 hornblende and triclinic feldspars seem to play an important part in the 

 composition of the rock. 



In the ridge opposite Spanish Spring Valley occurs a rock which may 

 be properl}^ classed as granite-porphyry, the groundmass being so exceed- 

 ingly fine-grained that the individual minerals cannot be recognized by the 

 unaided eye, but at the same time it carries well-developed crystals of all 

 essential ingredients of granite. In the rock under examination, quartz, of all 

 the normal constituents of granite, is the least readily recognized, although 

 here and there are some well-defined clear grains. Both monoclinic and 

 triclinic forms of feldspar are well developed, the latter frequently beauti- 

 fully stratified. The biotite is in general much better crystallized than the 

 hornblende, which is, however, thoroughly disseminated through the rock. 

 Under the microscope, the rock reveals the presence of considerable apatite, 

 and the quartz-grains seem poor in liquid-inclusions. Both of these obser- 

 vations in the microscopical analysis are of some interest, as Zirkel has 

 shown them to be characteristic of the later granites of Western Nevada. It 

 may be added that this rock, in its physical habit, resembles closely the sur- 

 rounding granite, but is somewhat more compact,' from the character of the 

 groundmass. 



Just east of Spanish Spring Valley, the granite-porphyry is penetrated 



