Glacier. A body of ice originating on land by recrystallization of snow, and showing evidence of movement by flowing. Gneiss. A coarse-grained metamorphic rock usually banded with streaks of darker, finer-grained rock. Granite. An intrusive igneous rock consisting essentially of sodium or potassium feldspar and quartz, often speckled with dark-colored minerals. Graptolite. Extinct marine organisms without known close living relatives, with small black sawblade-like chitinous hard parts preserved as fossils. Hematite. A steel gray or metallic grayish black or reddish gray mineral (Fe₂O₃) that is an important ore of iron. Hogback. A sharp-crested ridge formed by a resistant layer of steeply dipping rock. Huebnerite. A heavy reddish brown mineral (MnWO₄) that is a major ore of tungsten. Igneous rocks. Rocks formed by solidification from a molten state, either at the surface (extrusive) or below the surface (intrusive). Intrusive rocks. Igneous rocks formed when molten rock material solidifies without reaching the surface. Joint. A fracture in the rock, along which no discernible movement has taken place. Kerogen. Solid bituminous material in oil shales. Laccolith. A lens-shaped mass of igneous rock intruded into layered rocks. Lava. Fluid or molten rock such as that which issues from a volcano. Lode. A rock mass, often a vein, containing valuable minerals. Massif. A mountainous mass that has relatively uniform geologic characteristics and which may embrace a number of peaks. Mesa. A flat-topped mountain bounded on at least one side by a steep cliff. Metamorphic rock. Rock formed by alteration of pre-existing rock, especially by great temperatures and pressures. Mollusk. Any one of the large group of invertebrate animals which includes the snails, clams, octopuses, squids, and their extinct relatives. Molybdenite. A soft bluish gray, metallic mineral (MoS₂) that is a major ore of molybdenum. Monocline. A steplike fold in otherwise horizontal or gently dipping rock layers. Moraine. An accumulation of unsorted rock material built up by the action of glacier ice. Native gold. Gold occurring in nature uncombined with other elements. Peneplain. A land surface worn down by erosion to a nearly flat or broadly undulating plain. Petzite. A heavy black or steel gray metallic telluride ore of gold and silver (Ag₃AuTe₂). Placer. A sand or gravel deposit containing particles or nuggets of gold or other heavy valuable minerals. Plateau. An elevated, comparatively flat surface of land, usually larger than a mesa, sometimes composed of many mesas, and often dissected by deep stream valleys. Porphyry. An igneous rock, usually intrusive, which contains conspicuous large crystals in a fine-grained matrix. Pyrite. A brass-yellow metallic mineral (FeS₂) that is an important source of sulfur. It is commonly known as fool’s gold.