The Educational Series of Rock Specimens, Collected and Distributed by the United States Geological Survey

abundant acid aggregate albite alteration analysis andesite angles apatite appears augite basalt beds biotite brown calcite carbonate of lime cement chemical chlorite clay cleavage color colorless composed composition constituents contains COUNTY crossed nicols crystalline crystals dark deposited DESCRIBED BY J. S. diabase double refraction epidote eruptive extinction feldspar fragments gabbro Geological Survey glass gneiss granite green groundmass hornblende hypersthene igneous rocks inclusions iron irregular grains J. S. DILLER lamellæ lava limestone magma magnetite mass material metamorphic mica microcline microscope mineral minute muscovite nephelite nodules numerous occurs olivine oolite optical original orthoclase oxide parallel particles pebbles phenocrysts plagioclase plane plates pleochroism portion Price 15 cents prismatic prisms pyroxene quartz rhyolite sand sandstone sanidine schistose schists seen serpentine shale siliceous SiO2 sometimes Specific gravity spherulites structure substance surface thin section tion twinning U. S. Geol United States Geological usually volcanic zircon