CHAPTER II. 



VOLCANIC ROCKS. 



For so small an area the variety of volcanic rocks is great, yet there is a 

 marked similarity between the individual crystals of the same mineral species 

 wherever they occur, with some few exceptions, which links the various kinds of rocks 

 together and suggests the possibility of a common source. I^fevertheless, the differ- 

 ence between them in composition, structure, and physical appearance is suflicient 

 to establish their individuality. They have been divided into three groups — andesite, 

 rhyolite, and basalt — and have been considered in the order of their relative impor- 

 tance in the field. • 



ANDESITE. 



pyroxene-andesite (augite-andesite). —(a.)' The rock forming Richmond Mouutain is a dense 

 porphyritic lava, for the most part with a reddish purple homogeneous groundmass, 

 rich in macroscopic crystals of iieshcolored feldspar, the largest 4 or .'5""" long, without 

 distinct cleavage, and having a microtine habit; long black prisms of hornblende, with 

 very perfect prismatic cleavage and less noticeable pyroxene crystals. The dense 

 purple variety is in most every case parted or jointed in nearly horizontal planes. 

 A dark bluish black variety, with a resinous oily luster, occurs in compact masses 

 without fissile structure, and appears to pass insensibly into the purple rock. At 

 Trail Hill, the most northern spur of Richmond Mountain, the same rock traced 

 continuously from the main portion is vesicular and is rich in trittymite. A few 

 huiidret! yards to the south a compact fissile exposure shows a more crystalline 

 development and is exceptional. 



Under the microscope thin sections from various parts of the body have essen- 

 tially the same character — a gray, also yellowish to reddish gray, groundmass, com- 

 posed of coloi'less or yellowish brown glass, very rich in feldspar microlites, augite 

 prisms, and magnetite grains, with marked flow structure; abundant i)henocrysts of 

 zonally built plagioclase feldspar, with and without polysynthetic twinning, the 



' Since the first determination of these roclcs Tvas made, a separation and optical and chemical 

 analysis of the pyroxenic constituent of the Richmond Mouut.iiu andesite have been made and pub- 

 lished in the "Notes on the volcanic rocks of the Great Basin," by Arnold Hague and J. P. Iildings 

 (Am. Journ. Sci., Vol. xxvii, June, 1884, p. 458). This showed that the greater portion of the pyox- 

 ene belongs to the orthorhombic species nnd has the composition of hypersthene. It is therefore 

 more correct to place them under the hend of pyroxene-andesites, though thiy Tvere first termed 

 "augite" audesites, those from Riclunond Mouutain belonging to the hornblende-bearing variety, 

 348 



