ba a 
Hague and Iddings—Rocks of the Great Basin. 461 
feldspar. The rhyolites have been so well discussed and 
iste 4 i in the reports of the Fortieth Parallel Exploration 
and their microscopical characters described in such great detail 
by Zirkel that very little need be said within the limits of 
this paper except to give their subdivision and to point out 
some of their relations to other rocks. 
All the rhyolites of the Great Basin may be classified under 
one or the other of the following heads : 
Nevadite, 
Nevadite-—This rock is characterized by an abundance of 
porphyritic crystals imbedded in a relatively small amount 
of groundmass. It bears a strong superficial resemblance to 
granite, produced, as von Richthofen says, “ by the similarity 
of color, which is of light shades of gray and red, and by some 
affinity in mineral composition.” This resemblance, however, 
does not hold in a strict lithological’ sense, as the nevadites 
