190 H. B. PATTON TOPAZ-BEARING BHYOLITE, UTAH 



development must have been contemporaneous with that of the topaz, or 

 at least with the later stages of the topaz development, as they ma}' be 

 noticed growing upon the topaz and partially or completely inclosed in 

 its mass. They are usually very minute, one or two millimeters or even [ 

 less, and then may be very numerous ; rarely they attain to a diameter of 

 half a centimeter. They are perfectly black, highly lustrous, and very 

 thin and fragile. They occur not only in the lithophysal cavities, but 

 are also abundant attached to the rough opaque topaz crystals embedded 

 in the solid rhyolite. 



GARNET 



The occurrence of garnet at N"athrop, Colorado, in the cavities of rliyo- 

 lite was described by Cross in the j^aper above quoted.^^ They were de- 

 scribed as occurring in lithophysse associated with topaz, and were found 

 b}^ analj'sis to be spessartite. The largest crystals found, according to 

 Cross, measured 1 centimeter. It may be noted in passing that a speci- 

 men of the Nathrop garnet now in the collection of the Colorado School 

 of Mines measiires somewhat more than this, namely, 16 millimeters. 



At the locality which I have designated as Topaz mountain only one 

 very small garnet was observed, but at a jooint over the mountain to the t* 

 westward, some three or four miles from the rather limited area over I 

 which topaz crystals are al)undant, the writer was fortunate enough to \' 

 find a few very beautiful garnets of unusual size. The rock is a lithoidal * 

 rhyolite with quite marked flow structure that M'eathers into thin slabs 

 and that contains small irregular cavities. On the exposed surface the 

 cavities did not apj^ear to be lithophys^; but, as already mentioned, 

 weathering xery often destroys on the exposed surface all trace of the 

 lithophysal structure, and as no means were at hand to blast the rock, this 

 point could not be determined. The garnets were scattered sparingly 

 over a surface some hundred feet wide by two hundred feet long. Most 

 of them were very rough, showing only a face or two, owing to the fact 

 that the cavities were too small to accommodate them. The largest and 

 best crystal measures one and one-half inches and has twelve trapezohe- 

 dral faces entirely or partially developed. The rest of the crystal bears 

 the rough imprint of walls of the cavity in which it grew. The luster is 

 extremely brilliant and the color a very dark brown, beautifuly streaked 

 with a light bronze brown. The crystal form is the usual trapezohedron 

 (311) with the merest trace of a dodecahedron. The crystals are stri- 



" American Journal of Science, vol. 31, 1886, p. 434. 



