W. Cross — Topaz and Garnet in Rhyolite. 437 



miles north of Sevier Lake and nearly the same distance 

 W.N.W. from the town of Deseret on the Sevier Eiver. The 

 mountain containing the topaz (" Thomas' Range " of Simpson's 

 Report) is isolated, in an arid region. It is about six miles 

 long from north to south, has a fiat top and precipitous slopes, 

 and consists of a white or grayish eruptive rock, an evident 

 overflow, with indistinct banded structure and many amygda- 

 loidal cavities in which the topaz occurs. A small fragment of 

 the rock, sent by Prof. Clayton, seems to be a normal rhyolite. 

 It contains small brilliant quartz and sanidine crystals lying in 

 a dull white matrix, much resembling the common Nathrop 

 rock. The sanidine exhibits a delicate but distinct bluish 

 color parallel to a plane which could not be certainly deter- 

 mined but which no doubt corresponds to the orthodome ^-i. 



The crystal forms shown by this topaz are given by Engel- 

 mann, as follows: exhibited by all I, ^- 2 , 0, 4-2, 2; by most 

 crystals, 2-?, 1, •£; by a few, 4, 2-1. From an examination of 

 a few loose crystals sent me by Professor Clayton, I can add 

 that i-% is sometimes developed, while is lacking entirely in 

 a few cases. 



These crystals from Utah are larger than those from Nathrop. 

 The base of the prisms is said to be rough and imperfect, only 

 the ends being clear. Associated minerals have not been ob- 

 served and details concerning the cavities are lacking. Nearly 

 all crystals so far found are colorless, though pinkish, wine yel- 

 low and blue crystals have been noticed. 



From various statements which have come to my notice it 

 seems highly probable that a portion of the topaz found in 

 different Mexican localities is also derived from eruptive rock. 

 In no case has this been actually proven, so far as I know, by 

 the production of specimens. 



This is, it is believed, the first known occurrence of garnet in 

 cavities of rhyolite. In the list of minerals known to occur in 

 this manner in rhyolite J. Roth* mentions quartz, tridymite, 

 sanidine, biotite, augite and topaz, the latter from Chalk 

 Mountain only. To this must be added garnet and fayalite. 

 The occurrence of the latter mineral very recently described 

 by Iddingsf from the Yellowstone National Park is very sim- 

 ilar to that of the Nathrop minerals. 



The mode of formation of the topaz and garnet is not fully 

 determinable, but it is evident that they are not secondary 

 products, like zeolites, but primary, and produced by sublima- 

 tion or crystallization from presumably heated solutions, con- 



* Allgemeine und chemische Geologie, Band ii, p. 215, 1885. 



f This Journal, July, 1885, p. 58. It has also been recently observed by Tenne 

 in obsidian from Cerro de las Navajas, Mexico. Zeitschr. d. deutschen G-eol. G-esell- 

 schaft, 1885, 613. 



