432 W. Cross — Topaz and Garnet in Rhyolite. 



Art. XLL — Communications from the U. S. Geological Survey, 

 Rocky Mountain Division. VII. On the Occurrence of Topaz 

 and Garnet in Lithophyses of Rhyolite ; by Whitman Cross. 



(Read before the Colorado Scientific Society, March. 1, 1886.) 



In this Journal for February, 1884, I described the occur- 

 rence of minute crystals of topaz in the small drusy cavities of 

 a coarsely crystalline rhyolite from Chalk Mountain, by Fre- 

 mont's Pass, Colorado.* This was then supposed to be the 

 first published description of the occurrence of topaz in such 

 a manner in any eruptive rock, and especially noteworthy in 

 one of probable Tertiary age, but it has since then transpired 

 that the beautiful garnets and the rarer associated topaz from 

 Nathrop, Colo., which have been sold in the mineral stores of 

 Denver for two or three years past, present a second occur- 

 rence and that published mention of the same had already been 

 made by J. Alden Smith in the biennial " Report on the Devel- 

 opment of the . . . . Resources of Colorado, 1881-2," p. 36. 

 Mr. Smith mentions a dike of an eruptive rock in the Archaean 

 on the east bank of the Arkansas river, opposite Nathrop 

 station on the Denver and Rio Grande R. R., Chaffee Co., and 

 says that " a large part of it is composed of cellular pumice of 

 a light gray color; In the cavities are beautiful crystals of 

 topaz. ... Associated therewith are many small dark red 

 garnets." In the Catalogue of Minerals (1. c. p. 157) topaz is- 

 mentioned "in trachyte, near Nathrop." While I had examined 

 many small specimens from Nathrop previous to the description 

 of the Chalk Mountain occurrence, the gangue rock was so 

 poorly represented by them that its nature as an eruptive rock 

 was by no means clear, and the meager and somewhat inaccu- 

 rate statements of Mr. Smith escaped attention. 



Last October I had opportunity of visiting the locality at 

 Nathrop, obtaining fuller information concerning the occurrence 

 of the rhyolite, and a suite of specimens illustrating the same, 

 together with the contained minerals. 



Mode of occurrence of the Rhyolite. — The rock alluded to by 

 Mr. Smith, occurs directly opposite the station of Nathrop, form- 

 ing a ridge about one quarter of a mile in length and 200 feet 

 in height, running parallel to the river on its eastern bank- 

 North of this ridge and separated from it by a stream bed is- a 

 second ridge of rhyolite, of somewhat greater extent. A third 

 and smaller ridge of the same rock occurs on the western bank 

 of the river, near the railroad, and in this are stone quarries 

 now in operation. The common trend of these ridges is about 



*Also in Bulletin 20. U. S. Geological Survey, 1885, p. 81. 



