ACCESSORY MINKRALS. 263 



Gray Fox and in the porous white tutl's .soiitli of IJichmond Mountain. 

 Microscopic individuals of" hrown and reddish l)ro\vn ;dhuiite h;ive l)een 

 deterniined, abnost invariably in an unaltered state, in andesitic pearlite, 

 Rescue Canyon rliyolite, and in other very jilassy varieties of rhyolite. 

 The determination of allanite by its optical and iTystidlonrapliie properties, 

 its separation l)y chenucal nnalyses, and its occurrence in widely sepai-ated 

 localities j)ro\e that the mineral may claim recognition as an accessory 

 constituent in recent volcanic rocks.' 



In addition to the above minerals it may be well in this connection to 

 mention two nonessential constituents occurring- in the pyroxenic lavas — 

 tridymite and quartz — which, although of interest from a petrogra])hical 

 point of view have almost no bearing upon the ultimate composition of the 

 original molten mass. Tridymite is easil}- recognized under the microscope 

 in the vesicular rocks of Richmond Mountain in thin tabular crystals lap- 

 ping over each other in the manner so frequently observed elsewhere. 

 These leaf-like crystals arrange themselves in clusters lining the cavities. 

 Identical occurrences of tridymite may be observed in similar jiyroxene- 

 andesites from other localities in the Great Basin, notably in this type of 

 l:i\-a in the Wahweah Range northwest of Richmond Mountain. 



ijuartz as an accessory constituent has been recognized in the l)asalts 

 from a number of localities and apparently bears no relation to the chemi- 

 cal composition, being quite as apt to be developed in the normal olivine 

 l)asalts as in the more siliceous flow.s. It is as characteristicalh' displaved 

 in the l)asic rock of Magpie Hill as in any other, occurring in isolated irregu- 

 larlv shaped grains encircled on all sides by minute augite cr\stals. Under 

 the microscoiK' they have all the appearance of being of ])rimarv origin. 

 Similar quartz grains have been described by Mr. Iddings" from New 

 Mexico and Arizona, their origin being referred by him to physical causes 

 attending an earlier stage of the magma. He regards the exceptional devel- 

 opment of the quartz in these basic rocks as compai-able to the crystalliza- 

 tion of fayalite in the lithophysaj of rhyolitic obsidian. Similar ([uartz 

 grains in basalts have been described by Mr. J. S. Diller, from the base of 



'Joseph P. IiUlings aiul Wbitnian Cross: Widespread occurrence of allanite as an accessory 

 constituent of many rocks. Am. .lour. Sci., Aug., 1885, vol. xxx, pp. 108-111. 

 'Bull. U. S. Oeol. Survey, No. G(!, 18!«). 



