186 H. S. tATTON TOPAZ-BEARllS^G EHYOLlTE, UTAH 



These opaque topaz crystals are not always nor usually single indi- 

 viduals, but clusters of two to many crystals grown together. In these 

 clusters one individual shaft is usually larger than the others that 

 branch out from it in different directions (see plate 13, figure 2). 



There are numerous transitions between these opaque crystals and the 

 transparent ones. This is due to the fact that an opaque crystal may 

 not be confined to the solid rhyolite, but may project into a free cavity. 

 In this case the projecting portion, usually an end, will have the custom- 

 ary terminations of the other crystals in the cavities and will likewise be 

 transparent and of a wine color. As a matter of fact, the opaque crystals 

 when taken out from a freshly broken rock are not of a gray color, but 

 are of a mingled gray and wine color. From the foregoing it would 

 appear that the two varieties are due simply to different conditions of 

 growth. 



Smooth opaque topaz. — These crystals are exactly analogous to the 

 last described variety, in that they have the same habit and about the 

 same color and in that they occiir imbedded in a solid rock. As crystals, 

 the chief distinction lies in the extreme smootlmess of the faces and in 

 the fact that they have usually perfect terminations on both ends. Single 

 crystals are very rare. Eather they occur in clusters similar to those 

 just described for the rough opaque variety, with many prongs branching 

 out from a heavier central shaft. Only a very few perfect crystals and 

 clusters were found, as, on account of the brittleness and the easy basal 

 cleavage, the prongs are extremely liable to be broken off. In one case 

 as many as twenty subsidiary prongs were counted on one cluster. The 

 forms are the same as those described for the rough opaque variety, 

 namely, the two prisms with the same relative sizes and the pyramid 

 (221). In this case, however, other forms, with faces barely visible 

 upon close observation, can be more readily made out. For instance, a 

 second pyramid and all three pinacoids. The orthopinacoid that may 

 thus be made out is a form that has not as yet been described from this 

 locality. 



Another important distinction is the nature of the rock in which these 

 smooth cr5fstals are implanted. They, were found in just two points 

 about 50 feet distant from each other, and in each case they were devel- 

 oped in a fragment of rhyolite tuff that had been caught up by the rhyo- 

 litic lava stream. The tuff is quite firm, very fine grained, perfectly uni- 

 form in texture, and of a gray color. The crystals have been freed by the 

 disintegration of the tuff and were found lying on the surface or inclosed 

 in the soil immediately below the fragment of tuff or embedded in a fine 



