180 H. B. PATTON TOPAZ-BEAEING RHYOLITE, UTAH 



tuff of var}dng thickness, and on this in turn a succession of lava flows, 

 beginning with andesitic tj'pes, or perhaps locally with andesitie breccias, 

 and ending with the more acid rhyolites. At the base of these rhyolite 

 flows is to be seen, locally developed, a thin flow of obsidian that may be 

 quite free from crystals or crystallization products or that may locally 

 develop small solid spherulites. 



The rocks that form the outlying foothills on the east and southeast 

 and that are referred to above as having a dark brown or red color are 

 mostly andesitic in tj^pe, but the color of the surface is usually much 

 darker than that of the freshly broken rock and is no safe indication of 

 the basidity of the rock. In fact, many of the rhyolites of the region 

 are remarkably dark colored on the weathered surface. 



The later rhyolite flows undoubtedly make up the bulk of the volcanic 

 rocks, but it is not easy to distinguish sharply between the flows, nor to 

 trace their lines of contact, nor to measure their thickness, especially in 

 case of the rhyolites. It would require more time than was at the dis- 

 posal of the writer and a study of more distant parts of the range to settle 

 these matters with certainty. It would seem, however, that in the imme- 

 diate vicinity of Topaz mountain there are four and perhaps five lava 

 flows. The latest of these flows is the one that more immediately con- 

 cerns us, in that it contains most of the topaz crystals. 



The Topaz-bearing Ehyolite 



This rhyolite is several hundred feet thick and forms the crest of the 

 range on the southeast face. It varies in color from nearly white to 

 light brown or brownish gray. It is a lithoidal rhyolite, never showing 

 a trace of glass. In places it appears massive and structureless, but at 

 other places flow structure is very pronounced, so as to cause the rock on 

 weathering to split into thin slabs. The flow planes stand at all angles, 

 being fully as apt to be vertical as horizontal, and in many places are 

 greatly folded and contorted. The rock is apparently not porphyritic, 

 although it may be seen upon close observation to contain numerous very 

 small crystals of quartz and of sanidine. Dark colored minerals are en- 

 tirely wanting. This absence of ferromagnesian minerals may often be 

 accounted for in such rocks by supposing that they have been removed 

 by percolating waters which leave the rock more or less rotted. In fact, 

 the rhyolite does appear to be more or less kaolinized when examined in 

 the hand specimen alone. Under the microscope, however, the feldspars 

 present a surprisingly fresh appearance and very little kaolin-like ma- 

 terial is disclosed. 



