. Bulletin of the Geological Society of America. Geology. GKAPHITE-BEARING QUARTZITE 387. Figure 9.—Photomicrograiih of sheared Granite, for Comparison with the Quartz-mica Schist. X Nicols. (X 20) Locality, 1% miles northeast of Hellertown, Allentown quadrangle. Specimen 6749. Shows microperthite and quartz, much shattered, and surrounded by secondary micaceous material (dark) ; a large grain of perthite on the left has resisted the force, but is trav- ersed above by a tiny fault-crack containing angular, fragments. Figure 10.—Photomicrograph of Graphite-bearing Quartzite. Ordinary Light. (X
. Bulletin of the Geological Society of America. Geology. GKAPHITE-BEARING QUARTZITE 387. Figure 9.—Photomicrograiih of sheared Granite, for Comparison with the Quartz-mica Schist. X Nicols. (X 20) Locality, 1% miles northeast of Hellertown, Allentown quadrangle. Specimen 6749. Shows microperthite and quartz, much shattered, and surrounded by secondary micaceous material (dark) ; a large grain of perthite on the left has resisted the force, but is trav- ersed above by a tiny fault-crack containing angular, fragments. Figure 10.—Photomicrograph of Graphite-bearing Quartzite. Ordinary Light. (X 20) Locality, three-fourths mile southwest of Vera Cruz, Allentown quadrangle. Specimen 7788. The pale mineral is quartz, the dark graphite ; carbonaceous dust spreads out from some of the graphite plates into the quartz, appearing to cause a bluish color in the latter. Figure 11.—Photomicrograph of basic Gneiss. Ordinary Light. (X 20) Locality, 1% miles southwest of Lower Saucon, Allentown quadrangle. Specimen 6876. The dark mineral is hornblende, the white oligoclase feldspar, the .gray band crossing horizontally below the center sericite with dark patches of augite ; there is no indication that the hornblende is an alteration product of the augite, and both are thought to have formed simultaneously in the course of feldspathization of a shale. Figure 12.—Photomicrograph of basic Gneiss shoioing rounded Zircons. Ordinary Light. (X 100) Locality, 3 miles northwest of Boyertown. The clear mineral is orthoclase feldspar, and several zircons are visible, standing out in relief, near the center of the field ; they are well rounded and of different Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Geological Society of America. [New York : The Society]
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