182 H. B. PATTON TOPAZ-BEARING RHYOLITE, UTAH 



The rock immediately adjacent to the lithophj'sse may be designated as 

 a lithoidal rhyolite, in that it is crystalline rather than glassy and has a 

 stony luster. It is entirely free, however, from the branching rod-like 

 feldspar aggregates and from the microspherulitic growths so character- 

 istic of the Obsidian Cliff occurrence, and never presents a porcelain-like 

 appearance, although in some places, where lithophys^e are absent and 

 flow structure is unusually pronounced, the rhyolite has a fairly smooth 

 fracture, owing to fineness of grain, approximating that of the porcelain- 

 like lithoidite of Obsidian cliff. On the contrary, this rock is very dis- 

 tinctly holocrystalline and has a very rough fracture. The small quartz 

 phenocrysts, 1 to 2 millimeters in diameter, have a smoky color, in 

 marked contrast to the clear white of the quartz crystals that line the 

 shells. 



Under the microscope may be seen in thin-section a few small pheno- 

 crysts of sanidine and of quartz, all in roundish grains. Biotite and 

 other ferromagnesian minerals are entirely missing. The groundmass 

 consists of fairly coarse but very irregular and even ragged edged indi- 

 viduals of sanidine and of quartz, the former apparently predominating. 

 There is a certain amount of grayish, dirty looking impalpable powder 

 that probably represents some alteration product, possibly kaolin. The 

 character of this groundmass, taken in connection with the groundmass 

 of other portions of the extensive rhyolite flow more remote from the 

 lithophysse, impresses the observer as indicating a devitrification product 

 of an originally glassy rock. 



In addition to the customary constituents of a rhyolite, this rock also 

 contains the mineral topaz, not merely in the cavities, but as an impor- 

 tant ingredient of the rockmass. A description of this topaz follows in 

 these pages under the heading "Bough opaque topaz." 



Weathering Features 



The effect of the desert climate on this rhyolite is very remarkable. In 

 the first place, very little soil and almost no vegetation is to be seen, ex- 

 cept in places where the prevailing winds tend to accumulate soil. As a 

 general thing, the rock is swept absolutely bare. Under the influence of 

 the .climatic conditions, the rock does not rot, but disintegrates into sand 

 that is blown away by the wind as fast as liberated. Naturally the first 

 parts to be thus attacked are the delicate walls of the lithophj^sse, or, in 

 case these are not present, little cavities are scooped out where soft or 

 more friable parts of the rock occur. Thus the rock surface becomes 



