98 GEOLOGY OF THE COMSTOCK LODE. 



ency to zonal structure in this slide, but several of the hornblendes are 

 twinned according to the ordinary law. Decomposition has set in to a 

 slight extent ; and in one or two cases the degeneration into chlorite may 

 be observed starting from the cleavage fissures of the parent mineral. Where 

 the masses of chlorite have reached any considerable size, particles of epidote 

 have developed near their centers. In a large proportion of the hornblendes 

 occur inclusions of the same kind mentioned under slide 413. A group of 

 these microlites is shown in Fig. 21, Plate III. Their disposition is the same 

 as in slide 413, but this section contains nothing which throws light on their 

 nature. 



There are numerous good-sized but roun'ded plagioclases in this slide. 

 Those which show an approximately equal angle of extinction on each side 

 of the twinning 2)lane, give angles of extinction which, in some cases, con- 

 siderably exceed 20°; no untwinned microlites were observed, and the 

 feldspar is probably labradorite. The feldspars contain a few fluid inclusions 

 of apparently primitive character, and are pierced by numerous apatite 

 needles. One or two fragments of hornblende are inclosed in feldspars, 

 but for the most part the feldspars are wholly free from that mineral. 



The groundmass consists mainly of feldspar microlites and granules, and 

 traces of fluidal structure are perceptible. An abundance of magnetite is 

 recognizable as such from its crystal form ; and associated with and pene- 

 trating it are many colorless apatites. The slide also contains one poorly 

 developed zircon. There is further a small amount of chlorite and epidote. 

 Most of the former is concentrated in an excellent vein. 



Except in the matter of inclusions, this rock bears a strong resemblance 

 to an andesite ; its groundmass, however, is less microlitic and the porphyritic 

 feldspars have not the sharp development almost invariably observable in 

 andesites. Its occurrence as a mass little more than a foot cube, embedded 

 in porphyritic diorites of an ordinary variety, forbids the supposition that it 

 is a volcanic rock. A portion of the slide is shown in Fig. 26, Plate IV. 



Slide 27S. Ophir Ivuvine, south side. 



A second fresh porphyry. — Tliis rock strougly rescmblcs 421 in most respects, 

 but the hornblendes are noteworthy. They are unusually solid, often show- 



