. Bulletin of the Geological Society of America. Geology. QUARTZ-MICA SCHIST 383 shown toward the igneous rocks of the region, which is discussed in the following paragraph. The quartz-mica schists are frequently in contact with granite, and in every case the field relations show that the latter is the subsequent rock. It either cuts across laminae or forces them apart, giving rise to handed rocks, such as the ones illustrated in figures 7 and 8. On the other hand, sheared granites, observable at several places in the region, present a totally different picture. The quartz and feldspar grains
. Bulletin of the Geological Society of America. Geology. QUARTZ-MICA SCHIST 383 shown toward the igneous rocks of the region, which is discussed in the following paragraph. The quartz-mica schists are frequently in contact with granite, and in every case the field relations show that the latter is the subsequent rock. It either cuts across laminae or forces them apart, giving rise to handed rocks, such as the ones illustrated in figures 7 and 8. On the other hand, sheared granites, observable at several places in the region, present a totally different picture. The quartz and feldspar grains show progres- sively undulatory extinction, then shattering, a good example of which is shown in figure 9, then alteration to sericite, the final result being a pseudo-porphyritic mass of compact sericite with wavy and irregular lamination, dotted with rounded fragments of the original Figure 8.—Quartz-mica Schist shotcing Invasion hy Granite. (X %) Locality, 5 miles west of Boyertown. The layers of schist at the hottom are con- torted, and have evidently been practically melted by the granite, which permeates them. These rocks are the opposite of the quartz-mica schist in every respect. Quartz is small in amount, sillimanite and garnet absent, and zircon angular. Across the lamination the variation is gradual; along the strike rapid. And the igneous rock grades into the schist, never cutting sharply across nor penetrating it along laminae. MAGMATIG MODIFICATION As far as can be determined, feldspar is absent or present only in very subordinate amount in the normal schist. But where emanations from the granite magmas have acted on the rock, feldspar has been extensively introduced; it is chiefly orthoclase showing microperthitic intergrowth of oligoclase. This feldspar impregnates the rock more or less uniformly, surrounding the grains of the normal constituents. The lamina? may be but slightly separated, moderately deformed, and obscured to a negligible. Pleas
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
