Bulletin of the Geological Society of America, Vol. 12 (Classic Reprint)

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Excerpt from Bulletin of the Geological Society of America, Vol. 12Augite in small colorless or pale green crystals occurs at times. The amount, however, never seems to be great. The mineral is bounded by and (100) in the prismatic zone, as usual. Cleavage after (110) is present, but not well developed. Twins afier sometimes polysynthetic, are found. Extinction in sections near (010) is 42 degrees. The augite seems to alter more readily than h orn blende to chlorite, calcite, and epidote, so that the maximum amount of augite can never be ascertained. It seems certain, however, that this mineral plays a less important part in these rocks than the hornblende.Magnetite is always present in considerable quantity in both phenocrysts and groundmass. It occurs in well defined octahedrons, sometimes twinned after the Spinel law. The mineral decomposes to hematite and limonite. In advanced stages of alteration the presence Of leucoxene rims around the surfaces, as well as along the parting planes, proves that the mineral is titaniferous. Occasionally the shape of the crystals and amount of leucoxene is such as to indicate ilmenite, but the fact that the substance is so strongly magnetic bespeaks magnetite.Apatite in short hexagonal prisms, terminated evidently by base and pyramid, is generally present in and near the magnetite.A mineral having the properties of zircon is found very sparingly.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.