HHYOLITK. 379 



whose nature is indeterminable. The macroscopic quartz crystals have colorless 

 glass inclusions, which are for the most part spherical, a few having the form of nega- 

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

 rhombohedral cleavage. 



Rescue canyon Rhyoiite.-The second variety of rhyolite found in the district, thin 

 sections 162, 165, has many points of resemblance in microscopical habit to that just 

 described. It is, however, richer in phenocrysts, which under the microscope are 

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

 sions except a few glass dihexahedrons, 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, there are no 

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

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

 striated plagioclase, some with angles of extinction corresponding to labradorite. In 

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

 which this variety of rhyolite resembles that first 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 magnetite 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 which mark 

 its fluidal structure and give it a red color. 



Banded Rhyolite. The third variety differs from both the others and is in some 

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

 scarcity of macroscopic quartz. The four thin sections prepared, 174, 173, 169, 1(58, 

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

 as the same rock and described 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 part of which is sanidine, which is with 

 difficulty distinguished from quartz except by optical tests. Several sections of saui- 

 dine, with quadratic form and right-angled cleavage, remain dark when revolved 

 between crossed nicols, and give interference figures like crosses that are 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 than 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 composed of quartz grains, un- 

 striated feldspar, and microscopic spherulites, with many curved microlites which con- 

 sist of strings of transparent grains with a rather high index of refraction. Besides 

 these there is a little mica and magnetite. Thin section 169 is similar in the char 

 actor of its groundmass, which, however, is less coarsely crystalline and has a more 



