PETROLOGICAL ABSTRACTS AND REVIEWS ag fy] 
Patron, H. B. “Topaz Bearing Rhyolite of the Thomas Range, 
Utah,” Bull. Geol. Soc. America, XTX (1908), 177-92. 
Discusses the environment and conditions of origin of these rather 
well-known topazes. 
The topaz-bearing rock is a lithoidal rhyolite several hundred feet 
thick, the last of a series of five flows. Flow structure appears but 
locally and although the rock shows no solid spherulites, well-developed 
lithophysae are relatively abundant. These are lined with crystals of 
quartz, sanidine, garnet, specular hematite, topaz, and bixbyite. Topaz 
is also an important ingredient of the rock mass and the author describes 
the three varieties found. 
1. Transparent topaz, which always occurs in small cavities in the - 
rhyolite and was formed after the lithophysae were complete. 
2. Rough opaque topaz, which abounds in the solid rock as well as in 
the cavities. The crystals are longer than the transparent variety, 
gray in color, with ends roughened as though subjected to the action of a 
solvent. The interior of these crystals is crowded with sharply defined 
quartz grains. 
Forms transitional between groups 1 and 2 are found, from which it 
would appear that the two varieties are due simply to different conditions 
of growth. 
3. Smooth opaque topaz, which is analogous to the rough opaque 
variety in habit, color, and occurrence. This variety has been found 
in but two places in this region, in each case having been developed in a 
fragment of rhyolite tuff which had been caught up in the rhyolite 
streams. ‘These crystals are characterized by extreme smoothness of 
faces and the usually perfect terminations at either end. 
The inclosing rock is quite similar in composition to the rhyolite in 
which it is imbedded. Thin sections show an aggregate of very irregular 
grains of quartz and of sanidine that vary in size from o.1 mm. down- 
ward. ‘The topaz crystals are perfectly normal and inclose countless 
minute and invariably sharply defined crystals of quartz which together 
make up about one-fourth the bulk. No other inclosures are found. 
The writer concludes from his study of these topazes: (1) that the 
period of quartz crystallization was nearly the same as that of the topaz 
formation, and (2) that the agencies which caused the development of the 
topaz in the cavities also caused its development in the rock mass, in 
which space was provided partially by the complete removal of the feld- 
spar and the recrystallization of the silica in definite crystals of quartz. 
This makes the opaque topazes not exactly foreign matter but an integral 
part of the rock. HarRotp E. CULVER 
