TRANSPARENT TOPAZ 185 



and need not be here descril)ed at length. Mr A. iST. Ailing, in the paper 

 above quoted, mentions the following forms : 



Braehy pinacoid (010), basal pinacoid (001), two prisms (110) and 

 (120), two braehy domes (021) and (041), and four pyramids (223), 

 (111), (221), and (441). 



There are other forms not as 3'et determined that would repay a care- 

 ful study, but it is not within the scope of this paper to dwell upon the 

 erystallographic relationships. 



The brilliant luster of these topazes has often been commented on, and 

 this brilliancy holds for the crystals that have long been exposed to the 

 weather as well as to those freshly quarried. It would seem as though 

 they are practically unaffected by ordinary weathering influences. 



Rough opaque topaz. — The topaz crystals of this region are not con- 

 fined to the lithophysal cavities, but also abound in the solid rock. In 

 this case they are not transparent, but relatively opaque and of a grayish 

 color. In form they are rather slender, being from three to five times as 

 long as wide. They are also considerably larger than are the colorless 

 crystals of the cavities. They are seldom much less than an inch in 

 length, and from this they vary up to two and one-half inches or more. 

 The ends are almost invariably rough or even ragged in appearance, as 

 though they had been subjected to the action of some solvent. Usually, 

 however, one can see a trace and N&cy often much more than a trace of 

 one or more terminal forms. The pyramid (221) is the one usually in 

 evidence in such cases and other forms subordinate to this. The prism 

 faces, on the other hand, are invariably present and well developed, Imt 

 are somewhat roughened by clinging quartz crystals. The unit prism 

 (110) is invariably very conspicuous, while the other prism (120) is 

 present only as two very narrow and inconspicuous strips along the 

 sharper prism edge. The interior of these opaque crystals is crowded 

 with quartz grains, as disclosed under the miscroscope in a thin-section. 

 These are not the very irregular and ragged-edged quartz individuals 

 that characterize the groundmass of the rhyolite, but are sharply defined 

 roimdish gTains, with a marked tendency to definite crystal forms, and 

 occasionally they are perfectly formed crystals with the prism and plus 

 and minus rhombohedron. In the one crystal investigated the quartz 

 inclosures make- up perhaps one-sixth of the entire bulk. Tliey average 

 about 0.05 millimeter in diameter, with extremes of 0.1 millimeter and 

 0.02 millimeter. Larger quartz crystals may be seen adhering to the 

 outside ; also thin leaflets of specular hematite. Apparently none of the 

 feldsjjar that makes up a large proportion of the groundmass of the sur- 

 rounding rhyolite is to be found within these topaz crystals. 



