384 GEOLOGY OF THE EUEEKA DISTEIGT. 



tiuitbim stiuctuio, composed of mk-roscopic grains of varying size, which pass into 

 cryptoerystalline portions. The flow structnre is most noticeable in the thin section 

 Avithont the aid of a lens. The phenocrysts are quartz and feldspar in fragments. A 

 cross-section of zii'con, o-l""" broad, shows only one set of prism faces and a good 

 cleavage parallel to the other, which is seldom met with in microscopic zircon crystals 

 (Fig. 10, PI. III.) This thin section is similar to those from the Pinto Peak rhyolite. 



Thin sections 206, 207, and 208 from contact with basalt on the slope and at 

 the base of the same spur show exactly the same kind of alteration as 193, 191, and 

 192. which have been already described. The glass, however, is brown and red, with- 

 out the black trichites, and there is only a trace of the bisilicates and of biotite. Ifo. 

 209 is a beautiful section of a reddish brown breccia formed of fragments of brown 

 glass almost free from microlites cemented together by a dark red ferrite- bearing 

 glass, rich in microscopic shreds of biotite, which is very abundant, together with 

 green hornblende in small fi-agments. Pyroxene is scarce, there is comparatively 

 little quartz, and there are about equal amounts of sanidine and plagioclase, besides 

 which are magnetite garnet and zircon. 



Of the remaining instances of altered pumice one from contact with basalt on 

 the end of the east spur of Hornitos Cone, 210, is of puridish brown glass, containing 

 portions with very different structures, being itself an intimate mixture of brown and 

 gray glass with numerous grains of magnetite and a great abundance of brown horn- 

 blende in fragments, and with more perfect crystals of strongly pleochroic hypers- 

 theue with a narrow dark border; besides biotite, quartz, and feldspar, of which 

 plagioclase is in excess. The relative amount of the bisilicates and mica is much 

 greater than in any of the pumices previously described, and with an excess of 

 triclinic feldspar approaches nearer to the composition of an audesite. The altered 

 breccia from the summit of the cone presents in thin section 211 a red<lish gray matrix, 

 bearing yellow, orange, and red fragments, which are found to vary greatly in micro- 

 structure. There are comparatively few and small phenocrysts, principally of quartz 

 and feldspar, with still less biotite. The groundmass is a glass, in places microfelsitic, 

 also si)herulitic, and passing from cryptoerystalline into mierocrystalline. Among the 

 fragments are several that appear to belong to basalt. 



Similar to the last is the coarse breccia from the east side of Black Canyon, three 

 sections of which exhibit the changes wrought by the adjacent basalt. In general 

 they are poor in phenocrysts, plagioclase being the most abundant, together with a 

 little biotite and pyroxene, and besides the variously modified glassy portions are pieces 

 of the same basalt. The glass of thin section 224 is filled with irregularly shaped fluid 

 inclusions with stationary bubbles, besides patches of gray polarizing particles and 

 numerous magnetite grains. In thin section 225 the fluid inclusions have diminished 

 both in size and number and the contorted flow structure of the individual glass frag- 

 ments has been reduced ni(ue nearly to straight lines and tt) a general parallelism 



