436 IF. Cross — Topaz and Garnet in Rhyolite. 



frequent. These never show any tendency to hemimorphism. 

 The crystals occur singly as a rule, but groups are not uncom- 

 mon. 



In the porphyritic rock of the northern dikes the topaz 

 occurs in very small colorless prisms, easily mistaken for quartz 

 when not closely examined. 



Concerning the color of the topaz it was noticed that nearly 

 all of them obtained from open cavities were colorless or of 

 the pale bluish tint, while those found on breaking open solid 

 masses were most frequently wine-3 ellow. As nearly all speci- 

 mens were obtained from fragments broken out by visitors, 

 within the past two or three years, it would seem that the 

 action of sun-light might have produced the change in color. 

 J. Roth mentions,* while considering the changes produced in 

 minerals by light, that von Kokscharow has noted the bleach- 

 ing of deep wine-yellow topaz from the Urals, to an impure 

 bluish white, on an exposure to sun-light, of some months' 

 duration. 



Relation to other occurrences. — The Chalk Mountain occur- 

 rence, already referred to, is analogous to the present case in 

 that the containing rock is a rhyolite and that the topaz occurs 

 in cavities, but in detail the instances differ. In the Chalk 

 Mountain rock the cavities are small and show no tendency to 

 the lithophysal structure, and sanidine is the most abundant of 

 the associated minerals, with some quartz and a little biotite. 



An occurrence in Utah seems to be much more closely re- 

 lated to that of Nathrop. The locality was discovered by 

 Henry Engelmann, geologist of Capt. J. EL Simpson's expedi- 

 tion across Utah in 1859, and is mentionecLin Dana's System 

 of Mineralogy. Engelmann states in the report of Simpson's 

 expeditionf that the topaz crystals were found loose on the 

 surface and that they ." apparently originated from one of the 

 trachytic porphyries in that neighborhood." He did not see 

 any in the supposed gangue and his statement, unaccompanied 

 by any description of the rock, has naturally been overlooked 

 or discredited. 



I am indebted to Prof. J. E. Clayton of Salt Lake City, who 

 has visited the locality in person, for information and for speci- 

 mens illustrating this occurrence.^: The locality is about forty 



* Allgemeine und chernische Geologie, Bd. i, p. 42. 



f Report of Explorations across the Great Basin of the Territory of Utah, etc., 

 by Capt. J. H. Simpson, Engineer "Department U. S. Army. Washington, 1876, 

 p. 324. 



\ Another reference to this locality upon information derived from Professor 

 Clayton has been made by Mr. G. F. Kunz in the Geological Survey publication, 

 "Mineral Resources of the United States, 1883 and 1884," by A. Williams, Jr., 

 in the article on Precious Stoues, p. 738. The locality is first mentioned, in a 

 very confusing manner, as if in the Pike's Peak region ; the locality is wrongly 

 stated as west of Sevier Lake ; and the date of Captain Simpson's expedition is 

 given as 1847. 



