. Bulletin . AFT passe à travers le lit de gneiss de couleur claire, à 15 pieds au-dessous de la coupe transversale de la veine. À ce point il est de 3 à 4 pieds d'épaisseur et descend vers le southéastat un angle d'environ 25° (seefig. 10). La roche est blanche et très lustreuse sur les fracturesurfaces parallèles à l'âge de la cale. En plus de la sericite quartzand, une pyrite à grain fin et rarement des prismes de type aiguille de corne-blende dispersent légèrement la roche. En sec-tion mince (spéc. 506) sous le thème de la cicroscope, la roche a des bandes distinctes en raison de la localisation partielle de la sericitein étroite parall

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. Bulletin . aft passes through the bed of light-colored gneiss about15 feet below the cross-cut to the vein. At this point it is 3 to 4 feet thick and dips towards the southeastat an angle of about 25° (seefig. 10). The rock is whiteand highly lustrous on fracturesurfaces parallel to the cleav-age. In addition to the quartzand sericite, a little fine-grained pyrite and rarely smallneedle-like prisms of horn-blende are sparingly scatteredtlirough the rock. In thin sec-tion (Spec. 506) under themicroscope, the rock showsdistinct banding due to thepartial localization of sericitein narrow parallel bands.Quartz, the dominant mineral, occurs in small rounded grainsassociated with feldspar(albite ?), which is usuallyclear and unstriated, thoughmutiple twinning may be seenin places. Small cubes ofpyrite and a very little chloritemake up the accessory min-erals. The rock exposed in the bottom of the shaft is a dark greenish-gray gneiss in which tlie banding is only imper- 503), the minerals identified in. 40 Feet Fig. 10.—Vertical section through H. V.shaft, Young American ^line. V, vein: A, decomposed gneiss; B, dark-colored gneiss;C, light-colored gneiss. fectly developed. In thin section (Spec order of their abundance were hornblende, feldspar, quartz, biotite, chlorite, magnetite, and a little carbonate. The hornblende is light green in color THE GOLD MIXES OF THE DISTRICT. 133 and has rather low interference colors. The cleavage is good but thecrystals are irregular in outline and contain occasional inclusions of theother minerals. Alteration to chlorite is frequently shown. Part of thefeldspar is twinned after the albite law. luit much of it is unstriatcd anddifficult to distinguish from quartz, which it closely resembles in refractionand birefringence. The biotite is light brown in color and strongly pleo-cliroic. Angular grains of magnetite are scattered through the slide anda little carbonate is present. The small amount of carbonate contained in the gneiss just

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