Reef. A moundlike limestone structure built in the sea by sedentary organisms such as corals. Rhyolite. A light-colored volcanic rock with quartz and feldspar as the principal constituents. Schist. A metamorphic rock characterized by parallel orientation of flat-grained minerals like mica. Sedimentary rocks. Rocks formed of fragments of other rock transported by wind or water, or formed by precipitation from solution. Sphalerite. An amber-yellow to black mineral (ZnS) that is an important ore of zinc. Stalactite. A cylindrical or conical deposit of calcite hanging from the roof of a cavern, formed by evaporation of water droplets containing calcium carbonate. Stalagmite. Columns or ridges of calcite rising from the floor of a cavern, formed by water containing calcium carbonate dripping from a stalactite. Stock. A mass of igneous intrusive rock that covers less than 40 square miles, has steep sides, and extends to an unknown depth. Tennantite. A metallic gray mineral that contains copper, iron, and arsenic, and is an ore of copper. Tetrahedrite. A brittle, dark gray to black, metallic mineral containing copper, iron, zinc, and silver. Trilobite. One of a primitive group of extinct marine crustaceans, related to crabs and lobsters, having segmented bodies divided by longitudinal grooves into three lobes. Unconformity. A surface separating layers of rock, formed by a period of nondeposition or erosion. Vein. A crack or fissure filled with mineral material, often with valuable ore minerals. SUGGESTED READING There are thousands of scientific articles and books on Colorado geology, and many new ones appear each year. Following is a selection of books and booklets which we believe will be most useful and interesting in extending your knowledge of the state’s geology. Donnell, John R., editor, 1960, GEOLOGICAL ROAD LOGS OF COLORADO. Rocky Mountain Association of Geologists, Denver. Itineraries for a number of geological trips along Colorado highways and byways. Eckel, Edwin B., 1961, MINERALS OF COLORADO, A 100-YEAR RECORD. U. S. Geological Survey Bulletin 1114. Emmons, S. F., Cross, Whitman, and Eldridge, G. H., 1896, GEOLOGY OF THE DENVER BASIN IN COLORADO. U. S. Geological Survey Monograph 27. The classic early treatment of the surface geology around Denver, with many historic illustrations. Hansen, Wallace R., 1965, THE BLACK CANYON OF THE GUNNISON TODAY AND YESTERDAY. U. S. Geological Survey Bulletin 1191. A readable account of this unusual national monument near Montrose. Hansen, Wallace R., 1969, THE GEOLOGIC STORY OF THE UINTA MOUNTAINS. U. S. Geological Survey Bulletin 1291. The eastern part of this range is in Colorado. Henderson, C. W., 1926, MINING IN COLORADO, A HISTORY OF DISCOVERY, DEVELOPMENT AND PRODUCTION. U. S. Geological Survey Professional Paper 138.