KUVOLITH. ;!7>) 



whose uature is iiidetenninable. Tlic macrosfopic quartz crystals have colorless 

 glass inclusions, wbicb arc for the most jmrt spherical, a few having the form of nega- 

 tive crystals. There are no fluid inclusions atid some of the quartzes show distinct 

 rhombohedral cleiivage. 



Rescue Canyon Rhyoiite.— The secoud Variety of rliyolitc found in tlie district, tliin 

 sections lfi2, 1G5, has many jwiufs of resemblance in microscopical habit to that just 

 described. It is, however, richer in phenocrysts, whicli under tlie microscope are 

 found to be angular grains and fragments of quartz, which are very free from inclu- 

 sions except a few glass dihexahedrous, the dark color of the quartz not being trace- 

 able to noticeable inclusions. There is also faintly polarizing sanidine, sometimes 

 indistinguishable from quartz except by optical tests. In this, also, tliere are no 

 inclusions to account for the slight opalescence seen in tlie crystals on surfaces at 

 right angles to the base. Besides sanidine there is a comparatively large amount of 

 striated plagioclas(>, some witii angles of extinction corresponding to labradorite. In 

 addition to these abundant and larger jihenocrysts and a small amount of biotite, in 

 which this variety of rhyoiite resembles that tirst described, there is a small per- 

 centage of pyroxene in fragments and crystals, partly altered; one or two fragments 

 of brown hornblende without dark border, and some larger luagnetite grains. There 

 is also an irregular grain of garnet and one of allanite. The groundmass is partly 

 crystalline, partly glassy and axiolitic, with much ferrite in fine particles whicli mark 

 its Huidal structiu'e and give it a red color. 



Banded Rhyoiite.— The third Variety ditt'ers from both the others and is in some 

 instances of rather doubtful nature, owing to the abundance of plagioclase and 

 scarcity of macro.scopic quartz. The four thin sections prepared, 174, 173, l(i!t, 108, 

 have numerous jtoints of resemblance and, though differing somewhat, maybe classed 

 as the same rock and descrilied as rhyolites. Thin section 168 is of a wholly crystal 

 line rock, in which the phenocrysts are quartz (with a few glass inclusions and less 

 frequently gas cavities) and feldspar, the greater jiart of which is sanidine, which is with 

 difficulty distinguished from quartz excejit by optical tests. Several sections of sani- 

 dine, with quadratic form and i-ight-angled clea\age, remain dark when revolved 

 between crossed nicols, and give interference figures like crosses that ai'e optically 

 negative. They have numerous irregularly shaped gas cavities, which are especially 

 abundant near the margin of the crystal. Some of the cavities have a thin coating 

 of fluid around their walls, and a few contain more liquid tlmn gas. In these the 

 bubble is movable. There is also plagioclase and a little biotite, the latter tilled with 

 magnetite and red oxide of iron. The groundmass is comi)osed of (jiiartz grains, un- 

 .sti'iated feldsiiar, and microsiropic spheiulites, witli many curved inicrolites which con- 

 sist of strings of transjtaient grains with a rather high index of refraction. Hesides 

 these there is a little mica and magnetite. Thin section 109 is similar in the char 

 ai'ter of its groundmass, which, however, is less coarsely crystallini' and lias a more 



