Amphibians—cold-blooded four-footed animals which have gills in youth and lungs in maturity (e.g., frog). Anhydrite—the mineral calcium sulfate, CaSO₄. _See_ Gypsum. Anticline—an arch-like fold in the rocks, with the beds dipping in opposite directions on the two sides. Aquifer—a water-bearing layer of porous and permeable rock. Aragonite—a form of calcium carbonate (CaCO₃). Archeozoic—the oldest known geological era; early Precambrian. Bedding plane—the plane of demarcation between two individual rock layers or strata. Calcite—a mineral composed of calcium carbonate, CaCO₃. Caliche—an accumulation of calcium carbonate, commonly white in color, in the soil profile. Cenozoic—the latest era of geologic time, containing the Tertiary and Quaternary Periods and continuing to the present time. Chert—dense, hard rock of very fine-grained silica, usually in nodular form. This material is also called flint. Concretion—a concentration, usually spherical, of mineral matter in sedimentary rocks, produced by deposits from solution; it is harder than the surrounding rock. Conglomerate—a sedimentary rock composed of rounded, water-worn gravel, usually mixed with sand, and cemented together by another mineral substance. Coprolite—the fossilized excrement of animals. Eolian—pertaining to the erosion and the deposits resulting from wind action and to sedimentary rocks composed of wind-transported material. Epoch—a subdivision of a geologic period, such as the Pliocene Epoch of the Tertiary Period. Era—a major division of geologic time. All geologic time is divided into five eras: the Archeozoic, Proterozoic, Paleozoic, Mesozoic, and Cenozoic Eras. Fluorescence—luminescence of a mineral during exposure to radiation (such as from ultraviolet or X-rays). Fluvial deposit—sediment deposited by streams. Formation—a rock unit useful for mapping and distinguished primarily on the basis of lithologic character. Fossil—any remains or traces of plants or animals preserved in deposits of a past geologic age. Geode—a hollow stone, usually lined or filled with mineral matter. Geologic age—the age of an object as stated in terms of geologic time (e.g., a Pennsylvanian fern, Cretaceous dinosaur). Geologic time—all time which has elapsed since the first known rocks were formed and continuing until recent, or modern, times. Geologic time scale—record of the divisions of earth history. Gypsum—a mineral, hydrated calcium sulfate (CaSO₄·2H₂O). _See_ Anhydrite. Hoodoo—a form produced by erosion of rock. Ice age—the Pleistocene Epoch of the Quaternary Period, Cenozoic Era; a time of extensive glaciation. Igneous rock—rocks which have solidified from lava or molten rock called magma. Joint—a fracture in a rock along which there has been no displacement on opposite sides of the break.