WEATHERING OF THE RHYOLITE 183 



pitted with sharp depressions or excavations of all sizes. On the bottom 

 of such large and small cavities it is not uncommon to find topaz crystals 

 lying. ISTot infrequentl}^ such crystals lie imbedded in a layer of sand 

 and may then have preserved their original wine color. Both colorless 

 and opaque crystals were found in such cavities. 



Further exposure to the weather causes the cavities to deepen and 

 enlarge and the rock becomes honeycombed. The enlarging cavities 

 unite by the wearing away of thin intervening partitions until they be- 

 come miniature caverns perhaps 6 or 8 feet across and separated by the 

 thinnest of walls. Eventually the steep mountain side presents the most 

 rugged and ragged appearance imaginal)le and is suggestive of a great 

 porous sponge. Unfortunately photographs of these rugged formations 

 were ruined, but a fair notion may be obtained from examining a small 

 piece of the rock. It is only necessary to imagine a mountain side com- 

 posed of this same material with all the details on a greatly enlarged 

 scale. Figure 2 of plate 12 is from a photograph of such weathered rock 

 fragment. 



■*&' 



Minerals occurring in the Cavities of the Ehyolite 

 general reference to the minerals 



As already mentioned, the occurrence of topaz crystals in the rhyolite 

 of the Thomas range has long been loaown and their mode of occurrence 

 has been briefly described. In addition to the topaz crystals, there are a 

 few other minerals that likewise occur in these cavities and that will 

 receive brief description later. These are specular hematite, garnet, and 

 bixbyite. 



TOPAZ 



Physical characteristics. — Heretofore the only topazes that have been 

 described as coming from the Thomas range have been the transjiarent 

 and usually colorless variety; hwt there are other topaz crystals that I 

 shall designate as opaque topazes that are of perhaps equal if not greater 

 interest to the petrographer. These latter are usually rather rough in 

 their crystal development; but there was found in one locality a smooth 

 and sharply crystallized variety of this opaque topaz that occurs under 

 sufficiently different conditions to justify a separate description. The 

 topazes of this region will be described, therefore, under the following 

 three heads : Transparent topaz, rough opaque topaz, and smooth opaque 

 topaz. 



