104 



GEOLOGY OP THE HIGH PLATEAUS. 



It will be noted that while chemical constitution and mineralogical com- 

 ponents are the basis of the larger and broader divisions, the texture may 

 here be employed to distinguish the secondary characters. 



Group I.— RHYOLITES. 



The microscopic characters of the hyaline rhyolites and some of the 

 liparites have been studied and analyzed in a most admirable manner by 

 Professor Zirkel, and described by him in the volume on Microscopic Pe- 

 trography in the series of Reports of the Survey of the Fortieth Parallel, to 

 which volume the i*eader is referred. 



II. TRACHYTES. The trachytic group is characterized chemically 

 by a high degree of acidity, but inferior in that respect to the rhyolites. Its 

 dominant minerals are orthoclase, with a subordinate amount of plagioclase. 

 It is distinguished mineralogically from rhyolite by the absence of free 

 quartz, by the greater abundance of plagioclase, and of the subordinate 

 minerals hornblende, magnetite, augite, and biotite. In its texture and 

 physical characters it is also well separated in most cases, showing a 

 tendency to develop the coarsely granular and porphyritio habitudes rather 

 than the hyaline and vitreous, though the latter are not wanting, nor even 

 extremely uncommon. This group is nearly as varied in character as the 



