360 GEOLOGY OF THE EUREKA DISTRICT. 



along the lines of cleavage. An exceptional occurrence of mica is found in the andesite 

 exposed southeast of Trail Hill (No. 91). It does not form macroscopic crystals, but 

 occurs in small, irregular patches, closely associated with the macroscopic augite, and 

 also in more or less regular plates, quite uniformly disseminated through the groundmass. 

 It is brown and has a strong absorption, showing a large angle between the optic 

 axis, and appears in so fresh a rock to be undoubtedly of primary origin. A similar 

 occurrence is noted in the exceptional ''augite-andesite" from Palisade Canyon, Cortez 

 Range, described by Prof. Zirkel. 1 The two rocks, however, appear quite different 

 both in the hand specimen and under the microscope. The latter is coarsely crystal- 

 line and contains plagioclase, quartz, hypersthene, and brown mica, while the former 

 has a microcrystalline groundmass with porphyritical crystals. 



Quartz phenocrysts are very rare. Two rounded grains of very pure quartz 

 without inclusions are found in thin section 87. An irregular grain containing some 

 fluid inclusions, with briskly moving bubbles, in thin section 86, exhibits a varying 

 optical orientation, plainly arising from unequal tension throughout the individual. 

 It is found in the groundmass of the holocrystalline varieties (91-97), as the last min- 

 eral to crystallize, forming a cement for the other constituents. It can be determined 

 optically as a positive uniaxial mineral. It contains numerous glass and gas inclu- 

 sions. Its outline is very irregular, as the quartz individual extends among the 

 neighboring feldspar grains for some little distance, producing an irregular patch of 

 quartz substance, which becomes alternately dark and light throughout its whole 

 extent, as the thin section is rotated between crossed iiicols a micropoikilitic structure. 



Tridymite is very abundant in the vesicular forms of this andesite, thin sections 

 90, 87, 88. It occurs as microscopic aggregates of hexagonal plates about 0-02 mm in 

 diameter, filling small amygdaloidal cavities and incrusting the walls of larger ones 

 with easily recognizable macroscopic crystals. Tridymite has been found by Prof. 

 Zirkel in the precisely similar rock froin the south bank of Palisade Canyon, Cortez 

 Range, 2 and in the rock from the same locality, 3 before noticed in connection with the 

 occurrence of anorthite. 



The groundmass of these andesites has the "felt-like" structure noticed by 

 Prof. Zirkel as characteristic of " augite-andesite." It consists of a colorless glass 

 base crowded with microlites of feldspar and augite, with minuter crystals of magi.et- 

 ite associated with the augite, besides more or less dark colored globulites of an 

 indeterminable nature, the whole generally showing a marked flow-structure. The 

 proportion of glass base to microlites varies in different localities on Richmond 

 Mountain. It is most abundant in the dark resinous variety (Nos. 77, 78, 79), 

 where it is nearly equal to the microlites in amount. The gray color in these 

 thin sections appears to be due to minute magnetite grains, together with augite 



T. Zirkel. Micro. Petro.. U. S. Kxpl. 40th Par., vol. vi, p. 227, No. 527. 

 *Op. cit. specimen No. 311. 

 >Op. cit. specimen No. 310. 



