362 GEOLOGY OF THE EUREKA DISTRICT. 



The iurtividuals sliow both the polysynthotic twiuniug of albite and of pericline, 

 besides the simple Carlsbad twiuniug, which is often shown by the outline of the 

 sections, but the stri;e are in many cases few in number, aud are sometimes altogether 

 wanting. 



The larger feldspar crystals are especially rich in inclusions, which are massed 

 in the center or arranged in concentric zones, or are scattered irregularly through the 

 crystal. A good example of the zonal arrangement is seen in thin section 107. The 

 zone of inclusions in every case consists of minute particles of glass carrying globu- 

 jites and possibly gas bubbles, so densely crowded as to exceed in amount the inclos- 

 ing feldspar substance; when occurring scattered their form is seen to be in some 

 cases very irregular; in others rectangular, with the edges parallel to the outlines of 

 the feldspar crystal. In thin section 102, there are brown and gray globulitic glass 

 inclusions bearing augite microlites, besides which are isolated colorless glass inclu- 

 sions with gas bubbles, and an occasional microlite. There are also inclosures of the 

 groundmass and of the associated microlites. The smaller crystals are much freer 

 fi-om inclusions. The lath-shaped feldspar microlites forming the groundmass are 

 unevenly terminated and twinned in two or three stripes; the angle of extinction is 

 in general low, sometimes leaching the limit of labradorite, to which species they seem 

 to belong in part, though it is probable that a less basic species is also present. 



Pyroxene is abundant both as macroscopic crystals and as microlites in the 

 groundmass, its crystals are prisms, frequently very long and slender, with the prism 

 zone well developed; the pinacoidal faces are nuich larger than the prismatic ; the cleav- 

 age is poor, and there are many irregular fractures. The twinning is that ordinarily 

 met with. The pleochroism of the hypersthene is strong, but varies greatly among 

 the individuals in one and the same rock section, in some cases being scarcely per- 

 ceptible. The absorption and pleochroism are green parallel to c, light reddish brown 

 parallel to a. In sections at right angles to the vertical axis the colors are, yellow 

 parallel to a and grayish purple parallel to b, that is c=green, a=liglit reddish brown 

 to yellow, b=grayish pui-ple. Sections apparently in the same cry stallographic posi- 

 tion vary greatly in their degree of coloring. They are poor in inclusions, of which 

 the most characteristic are magnetite grains, apatite needles and glass. There is around 

 most of the augite crystals a narrow border of augite grains of final crystallization, 

 which also surrounds the black border of hornblende and magnetite as previously 

 described; some individuals are entirely free from it, and a very few have a partial 

 black border like hornblende. Fig. 2, PI. in. It is especially noticeable in thin 

 section 104, where of two pyroxene crystals almost in contact one, an augite, has a com- 

 plete border of magnetite, partially altered to red oxide, while the other, a hyper- 

 sthene, has no border whatever. The decomposition of the pyroxeue results in the 

 same yellow fibrous mineral mentioned under the Richmond iVIountain andesite. The 

 granular augite border and the smaller augite crystals and microlites in the ground- 



