X 



188 H. B. PATTON TOPAZ-BEARING RHYOLITE, UTAH 



It woiUd seem very clear from a study of these smooth opaque topazes 

 as well as of the rougher variety that the period of quartz crystallization 

 was practically identical in time, or nearly so, with that of topaz forma- 

 tion. The vapors or solutions, or both, that were responsible for the 

 development of topaz in the cavities were also responsible for the develop- 

 ment of the same mineral in the solid rock. In the latter case room had 

 to be made for the crystal, which was partly accomplished by the com- 

 plete removal of the feldspar and by the recrystallization of the silica into 

 definite quartz crystals. In describing an occurrence of topaz and garnet 

 in lithophysae in a rhyolite at Nathrop, Colorado, Mr Whitman Cross 

 says : 



"The mode of formation of the topaz and garnet is not fully determinable, 

 but it is evident that they are not secondary products, lilie zeolites, but 

 primary, and produced by sublimation or crystallization from presumably 

 heated solutions, contemporaneous or nearly so with the final consolidation of 

 the rock."" 



If this is true of the topaz crystals found in the lithophysal cavities — I 



and to the writer this seems to be indisputable — then it must also be true j 



of the other topaz crystals scattered throughout the mass of the rhyolite. | 



This would make these opaque topazes not exactly foreign matter, but an i 



integral part of the rock. i 



In the formation of both the rough and smooth opaque topaz crystals ' 



the material of which the inclosing rock was composed necessarily formed ; 



a serious hindrance to the growth of the crystal. In the case of the tuff, j 



the crystals must have been formed in the solid rock under the influence ' 



of the hot lava and by means of the same vapors and solutions that caused i 

 the development of the crystals in the rhyolite, and in their formation 

 they must have made place for themselves through the removal of the 

 rock ingredients by solution or by partial recrystallization. T\niere the 

 constituents of the rock were very fine and uniform, as in the case of the 



tuff, they offered less resistance to the recrystallizing agencies, with the I 



result that the topaz crystals were able to assume perfect forms. | 



In this connection it may be noted that the tendency to the develop- i 



ment of the two prism forms, especially of the unit prism, was much j 



stronger than in the case of the other forms, so that these forms are I 



always more or less in evidence, while the tendency to develop terminal j 



forms was not strong enough to overcome the resistance in case of the ; 



coarser grained rhyolite, but was sufficient to overcome the weaker resist- | 



11 American Joui-nal of Science, vol. 31, 1886, p. 437. 



