240 GEOLOGY OF THE EUREKA DISTRICT. 



Richmond Mountain.— The inoimtaiii lies iu a region of profound disturb- 

 ance and dislocation. Innnediately to the west the depressed block of 

 Spring Hill sinks beneath the plain, while to the south the broad elevated 

 mass of County Peak rises abruptly above the lavas. The Pinto fault passes 

 beneath Richmond Mountain and apparently connects with the Rescue 

 fault, the great line of displacement which separates the County Peak 

 block from the Diamond Range, but all structural features are obliterated 

 by the pyroxene-andesite lava flows. The culminating point of Richmond 

 Mountain, situated near its southern end, attains an elevation of nearly 

 2,000 feet above Diamond Valley. An abrupt wall, 800 feet in height, 

 forms the southern end, and from its summit the mountain falls away to 

 the north for nearly tlxree miles, with an avei'age slope of about 14°. 

 Across its broadest expansion, in an east and west du'ection, the mountain 

 measures three miles. For such an accumulated pile of lavas it presents a 

 uniform, monotonous appearance, relieved by occasional shallow drainage 

 depressions flowing northward, inclined with the natural slopes of its lava 

 ridges. Its geological position with reference to the Carboniferous beds 

 of the Diamond Range on the east, the Devonian on the south, and the 

 Silurian and Cambrian beds on the west is shown iu cross-section A-B, 

 atlas sheet xiii. 



Richmond Mountain is almost wholly made up of pyroxene-andesite, 

 the prevailing colors of which are dark grayish purple varying to 

 bluish black. In crystalline structm-e the rock varies from a micro- 

 crystalline groundmass to one rich iu glass base, porphyritic crystals 

 of light colored feldspars characterizing the rock through all degrees of 

 crystallization. This range in crystallization produces marked variations 

 in physical features, the lavas changing within short distances from a 

 highly vesicular rock with angular fracture to a compact one weathering 

 with rounded outlines. Varying proportions of the poi'phyritic constitu- 

 ents are found in all the rocks from the holocrystalline to those rich in 

 glass base. Hypersthene is the prevailing pyroxenic mineral, always 

 accompanied, however, b}' more or less augite. The feldspars are anorthite 

 and labradorite. In addition to the essential minerals, well developed 

 porphyritic crystals of hornblende occur in the less basic lava, but nowhere 



