370 GEOLOGY OF THE EUREKA DISTRICT. 



is strongly pleochroic, green and ligbt reddish brown, similar in all points to that of 

 the pyroxene-andesite. Its decomposition, which is the same as that already described, 

 has advanced farther than in the pyroxene-andesites and is illustrated in Figs. 5 and 

 !>, PI. in. Augite is found only in two thin sections from this locality, Nos. 56 and 57. 



Biotite is macroscopically the most prominent mineral in the quartzose members 

 of this series. It is in hexagonal plates of a dark brown color with strong absorption, 

 and is optically negative with a very small angle between the optic axes. It is 

 twinned as in the hornblende-mica-andesite. Quartz is not so abundant in the thin 

 sections as in the hand specimens and is always in rounded grains or angular frag 

 ments with a few glass inclusions. Magnetite, apatite, and zircon are common to all 

 the varieties of this pearlite. The apatite is like that found in the other audesites; a 

 fine example showing the terminations and a basal cleavage is represented in Fig. 8, 

 PI. in. 



The zircon crystals are not more numerous in this than in many other rocks 

 where there are found to be three or four crystals to a rock section, but their occur- 

 rence here in unaltered feldspars or isotropic glass renders them more than usually 

 favorable for study, and so a number have been drawn to show their crystal faces, 

 which were recognized by careful study in all possible lights and were drawn with the 

 aid of a camera. Owing to the high index of refraction of zircon the marginal faces 

 can not be as accurately determined as those near the center of the figures, and the 

 terminal planes of Figs. 15 and 20 being extremely minute could not be made out for 

 the same reason. It should also be remarked that the drawings are not mathematical 

 projections, because with the high magnifying power employed, in one instance 900 

 diameters, only a small part of a crystal is in focus at any one time, and a certain 

 amount of distortion necessarily follows. The figures represent, however, the sharp- 

 ness of the crystallization and will indicate the forms taken by the crystals. Besides 

 the short, stout crystals, from 0-05 to ()! """ long, more usually met with, there are 

 sometimes long, slender prisms reaching a length of 0-37 mm and terminated at one or 

 both ends, Figs. 15 and 16, PI. in. The form of Fig. 15 appears to be the two prisms, 

 ooP, ooPoo , the double pyramid or zirconoid 3P3, and the pyramids P and Pec ; that 

 of Fig. 16 ooP, cePx. , and 3 P3; and Fig. 17 o>P, ooPoo , 3 P3, with P or .P . Fig. 

 18 represents a very simple form, combining a prism with a pyramid of the opposite 

 order. Fig. 19 seems to present both prisms xP, -Poo , the double pyramid 3P3, 

 and the two pyramids P and P- ; and Fig. 20 ooP, ooPao , 3P3, and two pyramids P 

 and Pec . The occurrence of similar microscopic zircons has been observed by the 

 writer in most all kinds of rocks, except the very basic, but more especially in the 

 mica-bearing varieties, with which mineral it is frequently in close association. 



In noticing the different varieties of this audesitic pearlite the description will 

 be confined to the series found in the vicinity of Dry Lake and the correspondence or 

 points of difference in the similar forms from the other localities will be mentioned in 



