ACCESSORY MINERALS. 263 



Gray Fox and in the porous white tuffs soutli of Richmond Mountain. 

 Microscopic individuals of brown and reddish brown allanite have been 

 determined, almost invariably in an unaltered state, in andesitic pearlite, 

 Rescue Canyon rhyolite, and in other very glassy varieties of rhyolite. 

 The determination of allanite by its optical and crystallographic properties, 

 its separation by chemical analyses, and its occurrence in widely separated 

 localities prove that the mineral may claim recognition as an accessory 

 constituent in recent volcanic rocks. 1 



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



/ 



mention two nonessential constituents occurring in the pyroxenic lavas 

 tridymite and quartz which, although of interest from a petrographical 

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

 original molten mass. Tridymite is easily 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 pyroxene- 

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

 lava in the Wahweah Range northwest of Richmond Mountain. 



Quartz as an accessory constituent has been recognized in the basalts 

 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 

 basalts as in the more siliceous flows. It is as characteristically displayed 

 in the basic rock of Magpie Hill as in any other, occurring in isolated irregu- 

 larly shaped grains encircled on all sides by minute augite crystals. Under 

 the microscope they have all the appearance of being of primary origin. 

 Similar quartz grains have been described by Mr. Iddings 2 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 comparable to the crystalliza- 

 tion of fayalite in the lithophysse of rhyolitic obsidian. Similar quartz 

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



'Joseph P. Iddings and Whitman Cross: Widespread occurrence of allanite as an accessory 

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

 'Bull. U. S. Oeol. Survey, No. 66, 18!K). 



