. Bulletin of the Geological Society of America. Geology. CRYSTALLINE LIMESTONE 379 Where it has been acted on by intrusive magmas the limestone has been considerably metamorphosed, minerals of the amphibole and pyroxene groups having been developed in large amounts. In some cases small bodies of amphibolite and pyroxenite with gneissoid texture have formed, the calcium carbonate having com]3letely disappeared (see figure 1). Further details need not be given here, as typical occurrences of this for- mation have been described in reports of the Pennsylvania Topographic and Geologic Survey Comm
. Bulletin of the Geological Society of America. Geology. CRYSTALLINE LIMESTONE 379 Where it has been acted on by intrusive magmas the limestone has been considerably metamorphosed, minerals of the amphibole and pyroxene groups having been developed in large amounts. In some cases small bodies of amphibolite and pyroxenite with gneissoid texture have formed, the calcium carbonate having com]3letely disappeared (see figure 1). Further details need not be given here, as typical occurrences of this for- mation have been described in reports of the Pennsylvania Topographic and Geologic Survey Commission.^ ORIGIN Assignment of a sedimentary origin to this limestone needs no justifi- cation. Quartz-mica Schist character This formation is typically a pale greenish gray rock composed of flattened lenses of quartz separated by films of sericite mica, through which extend blades of sillimanite. A specimen is shown in figure 2. This grades on the one hand into quartzite by the disappearance of the. Figure 2.—Quartz-mica Schist with Sillimanite. (X %) Locality, 1 mile north of Springtown, Allentown quadrangle. Specimen 9243, The sillimanite appears as white blades against the brownish gray quartz ; the upper part of the specimen is stained red—which appears dark in the photograph—by iron oxide i-e- sulting from the decomposition of pyrite. mica, and on the other into muscovite-schist as the quartz diminishes and the mica becomes more coarsely crystalline. In addition to tlie sillimanite several other minerals are occasionally present as accessory constituents, and may locally become abundant; the most frequent are apatite, biotite, garnet, ilmenite, pyrite, tourmaline, and zircon. 8 Frederick B. Teck : Preliminary report on the talc and serpentine of Northampton County. Topographic and Geological Surv. Penna., Rept. 5, 1911, pp. 12-23 ; Benj. L. Miller : Graphite deposits of Pennsylvania. Ibid., Rept. 6, 1912, pp. Please note that these images are extracted from s
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