380 GEOLOGY OF THE EUREKA DISTRICT. 



marked flow structure, produced by variations in microstractnre and in the minute 

 particles of coloring matter. It is poor in phenocrysts. 



The two remaining thin sections (173, 174) are of somewhat doubtful character. 

 The groundmass is but partially crystalline, with yellow and colorless glass. It is 

 rich in grains of iron oxide, both black and red, and has a markedly banded structure, 

 as shown in Fig. 2, PI. viii. It is poor in phenocrysts, the greater number being 

 plagioclase, with marked zonal structure. They carry more glass inclusions than are 

 found in the plagioclase of the other rhyolites of the district. Biotite filled with red 

 oxide of iron is next in abundance, besides which there is a little apatite in compara- 

 tively large crystals, and one crystal of augite. There are numerous groups of color- 

 less grains of irregular shape, which appear to be tridymite. There is, indeed, a 

 close resemblance in some of its microscopical characters to certain forms of andesite, 

 while at the same time it seems closely allied to some forms of rhyolite. 



Rhyoiitic Pumice. Before describing the pumices it will be interesting to notice 

 the rhyolite of Purple Hill because of its easily traced connection with the adjoining 

 pumice and pearlite into which it is seen to pass. Thin section 176 is from rhyo- 

 lite on the summit of the hill ; No. 177 is from the same at the northeast base of the 

 hill where it passes into pumice. No. 180 is from denser pumice, almost pearlite, and 

 178 is from the dark compact pearlite. The first is light gray in thin section and has 

 a glassy groundmass filled with faintly polarizing particles and larger feldspar micro- 

 lites, together with numerous amygdules of tridymite. The phenocrysts are quartz 

 and feldspar, of which sanidine predominates over the plagioclase. There is a little 

 impure biotite and a fragment of pyroxene and some magnetite. In the next thin sec- 

 tion (177) the phenocrysts are much scarcer and the groundmass is a colorless glass 

 filled with gas cavities, some of which are spherical, but the majority are elongated, 

 spindle shaped, and drawn out to long tubes, that are much twisted and bent. There 

 are numerous six-sided microscopic mica plates and a smaller number of feldspar and 

 hornblende microlites. Much, if not all, of the opaque grains that are scattered in 

 patches through the groundmass is foreign to the rock and has filled cavities during 

 the grinding of the thin section. A more advanced stage is seen in thin section 180; 

 the phenocrysts are the same in character, but are more abundant, with a noticeable 

 amount of pleochroic hypersthene. The glassy groundinass is rich in spherical gas 

 bubbles and microlites of feldspar, hornblende, and biotite, with a small percentage of 

 trichites, which reach a greater development in the more perfect pearlite, thin section 

 178. The colorless glass of this rock bears a multitude of the most beautiful micro- 

 lites, consisting of colorless rectangular crystals of feldspar, brown hexagonal plates 

 of biotite, dark green prisms of hornblende, and curved trichites which appear opaque 

 under a low magnifying power, but are found to consist of a transparent fiber with 

 serrated edges or to be a string of disconnected globulites. They are grouped about 



