_Rejuvenated._--Any region which has been subjected to erosion for a greater or less length of time and then reelevated so that the streams are renewed in activity. _Rock._--Any extensive constituent of the crust of the earth, usually consisting of a mechanical mixture of two or more minerals, e.g. granite, shale. Less commonly a rock consists of a single mineral (e.g. pure marble), or of organic matter (e.g. coal). _Sandstone._--A sedimentary rock consisting of consolidated or cemented sand. Often shaly or limy. _Schist._--A rock that by subjection to heat and pressure and usually moisture within the earth has undergone a change in the character of the particles or minerals that compose it and has these minerals arranged in such a way that the rock splits more easily in certain directions than in others. It is a metamorphic rock derived from either sedimentary or igneous rock, more commonly the former. _Sedimentary Rocks._--Rocks formed by the accumulation of sediment in water (aqueous deposits) or from air (eolian deposits). The sediment may consist of rock fragments or particles of various sizes (conglomerate, sandstone, shale); of the remains or products of animals or plants (certain limestones and coal); of the product of chemical action or of evaporation (salt, gypsum, etc.); or of mixtures of these materials. Some sedimentary deposits (tuffs) are composed of fragments blown from volcanoes and deposited on land or in water. A characteristic feature of sedimentary deposits is a layered structure known as bedding or stratification. Each layer is a bed or stratum. Sedimentary beds as deposited lie flat or nearly flat, but subsequently they have often been deformed by folding and faulting. _Shale._--A sedimentary rock consisting of hardened thin layers of fine mud. _Silurian._--A period of the Paleozoic era of geologic time--in order of age, the third from the beginning of the era. _Slate._--A rock that by subjection to pressure within the earth has acquired the property of splitting smoothly into thin plates. The cleavage is smoother and more regular than the splitting of schist along its grain. It is a metamorphic rock nearly always derived from shale. _Soil._--The mantle of loose material resting upon bedrock, either in its place of origin or transported by water, wind, or ice. _Strata_ (or stratified rocks).--Sedimentary rocks which, by the sorting power of water (less often by wind), are arranged in more or less definite layers or beds separated by stratification surfaces. _Stratification._--The separation of sedimentary rocks into more or less parallel layers or beds. _Stratigraphy._--The branch of geologic science that deals with the order and relations of the strata of the earth's crust.