SMOOTH OPAQUE TOPAZ 187 



gray sand that lay in the bottom of a cavity of one of the tuff masses and 

 that resulted from the slow disintegration of the same. Nearly all the 

 perfect crystals were found in this last named place. Those in the soil 

 nearly always had some or most of the prongs broken off. It was impos- 

 sible to break a crystal iminjured from the solid tuff. 



Under the microscope a thin-section of this tuff disclosed an aggregate 

 of very irregular grains of quartz and of sanidine that vary in size from 

 0.1 millimeter downward. The rock is quite similar in mineral compo- 

 sition to the rhyolite in which it is embedded, but quite different in 

 appearance, both in the hand specimen and in thin-section. Thin- 

 sections cut through one of these crystals parallel to the basal pinacoid 

 and to the macropinacoid show a perfectly normal crystal of topaz in- 

 closing countless minute and invariably sharply defined crystals of quartz 

 that together make up something like one-quarter of the bulk. These 

 crystals measure from 0.03 millimeter to 0.005 millimeter. They contain 

 minutest dust-like particles that are apparently exactly like dusty par- 

 ticles in the tuff. These particles are sometimes black and opaque, some- 

 times reddish or brownish, and are more sharply defined in the topaz 

 crystals than in the rock. No other inclosures than the quartz crystals 

 occur in the topaz. 



The following chemical analyses of the rhyolite tuff in which the 

 smooth opaque topaz crystals occur and of one of the smooth opaque 

 topaz crystals were carried out in the chemical laboratories of the Colo- 

 rado School of Mines. The analysis of the tuff was made by J. W. 

 Whitehurst and that of the topaz crystal by J. H. East, both students of 

 the class of 1910. In case of the rhyolite tuff, it is readily seen from 

 the analysis that the tuff does not differ materially from a normal rhyo- 

 lite. The topaz analysis shows naturally an abnormally high percentage 

 of silica, due to the inclosed quartz crystals. A calculation based on this 

 excess of silica indicated that 18.78 per cent of the material of this 

 opaque topaz is inclosed quartz. 



Rhyolite Tuff Smooth opaque Topaz 



SiO, 74.50 SiOj 44.68 



ALO3 13.28 AI2O3 44.84 



FeoOs 1-50 NaoO trace 



CaO 1.46 KjO trace 



MgO trace H^O trace 



K2O 3.54 F 13.44 



Na,0 5.23 



Total 99.51 Total 102. 9G 



