734 DESCRIPTIVE GEOLOGY. 



Range to the south, where they form a low group of rhyolitic rocks rising 

 but a few hundred feet above the valley. Lying for the most part below 

 the level of the old lake basin, they have suffered considerably from denu- 

 dation, the retiring waters eroding them into the most rough and broken 

 outlines, and leaving traces of the more permanent water-levels in the still 

 remaining terraces. These rliyolites, as a group, differ considerably from 

 those in the Pah-Ute Eauge, being far poorer in secreted minerals and 

 revealing only small sanidins and both dark and colorless grains of 

 quartz. Under the microscope may be detected, in a few cases, small 

 flakes of biotite. Mineralogically, therefore, they present but little variety, 

 but physically they offer one of the most interesting bodies of rhyolite 

 in Western Nevada, showing the greatest possible differences in colbr, tex- 

 ture, and those peculiar structural forms so characteristic of rhyolite. In 

 color, while they have a prevailing reddish tint, they vary from deep brick- 

 red, through brownish and yellowish shades, up to pink and salmon, pro- 

 ducing the most marked and brilliant contrast, which is greatly heightened 

 by the almost entire absence of soil and vegetation. The cause of this 

 almost kaleidoscopic effect of color has been shown by Professor Zirkel to 

 be due to the presence of ferritic needles and oxide of iron, filling the micro- 

 scopical fissures in the groundmass after the development of the quartz and 

 sanidin. In texture, these rocks vary from earthy, porous types, through 

 infinite gradations and transitions, to compact masses, with a distinctly lith- 

 oidal character. In but few localities is the lamellated, banded structure more 

 strikingly shown than here, even to the unaided eye the parallel lines run- 

 ning for long distances like narrow layers in sedimentary clays and sands, 

 although much finer. But microscopically this structure is still more remark- 

 able, and Professor Zirkel has given with some detail the results of his exam- 

 ination.^ He estimates that in some cases these lines are so extremely dehcate 

 that twelve of them taken together would not measure in thickness more 

 than 0.03°^". 



The Mopung Hill eruptions are largely made up of breccia suspended 

 in a fine felsitic groundmass, through which break later flows of white and 



1 Microscopical Petrography, vol. vi, 181. 



