Joint System—a series of two or more sets of joints passing through a rock mass and separating it into blocks of more or less regular pattern. Mass-wasting—erosion caused chiefly by gravity. Mesozoic—the geologic era between the Paleozoic and Cenozoic Eras; the “Age of Reptiles.” Metamorphic rock—rock formed from igneous or sedimentary rocks that have been subjected to great changes in temperature, pressure, or chemical environment. Metamorphism—the process whereby rocks are changed physically by heat, pressure, or chemical environment into different kinds. Mineral—a naturally occurring inorganic substance possessing definite chemical and physical properties. Nodule—rounded lump of rock or mineral. Outcrop—the area where a particular rock formation comes to the surface. Paleontology—the science which deals with the study of fossils. Paleozoic—that era of geologic time following the Proterozoic and preceding the Mesozoic. Period—a basic unit of the geologic time scale into which the eras are divided, such as the Pennsylvanian Period of the Paleozoic Era. Permian—the seventh and last period of the Paleozoic Era. Pleistocene—the first of the two epochs of the Quaternary Period, and that which precedes modern time, known as the Great Ice Age. Pliocene—last and youngest epoch of the Tertiary Period of the Cenozoic Era. Proterozoic—youngest era of the Precambrian; follows the Archeozoic Era and precedes the Cambrian Period of the Paleozoic Era. Red beds—a general term for red sandstone, shales, etc., which appear to characterize arid periods in the past. Ripple marks—wave-like corrugations produced in unconsolidated materials by wind or water. Rock—any natural aggregate of mineral matter, usually consisting of a mixture of two or more minerals. Sandstone—sedimentary rock composed of cemented sand grains, usually quartz. Sediment—material that has been deposited by settling from a transportation agent such as water or air. Sedimentary rock—rocks formed by the accumulation of sediments. Shale—a sedimentary rock formed by the hardening of mud and clay and usually tending to split into thin sheets or layers. Silica—an oxide of silicon (SiO₂). Siliceous—containing or pertaining to silica. Silt—fine muddy sediment consisting of particles intermediate in size between clay particles and sand grains. Siltstone—a very fine-grained sedimentary rock composed of silt grains, and intermediate between shale and sandstone. Stratified rocks—sedimentary rocks; those formed in beds, layers, or strata. Stratum—an individual layer of rock formation. (Plural, _strata_.) Superposition, law of—in an undisturbed sequence of rocks younger beds overlie older beds. Syncline—a trough-like fold in the rocks, with the beds dipping inward on either side. _See_ Anticline.