234 



TERX UNITED STATES. 



Meander. To flow in serpentine curves. A loop in a stream. The term comes 

 from the Greek name of a river in Asia Minor, which has a sinuous course. Most 



,nis 



m flowing across plains develop meanders. (See PI. XXXVII, p. 177,) 

 Metaniorphism. Any change in rocks effected in the earth by heat, pressure, 

 solutions, or gases. A common cause of the metamorphism of rocks is the intru- 



igneous 



ged are termed 



metamorphic. Marble, for example, is metamorphosed limestone. 



Monzonite. An even-grained intrusive igneous rock intermediate in character 

 between diorite and granite. It resembles granite. 



Moraine. A mass of drift deposited by a glacier at its end or along its sides. 



Oil pool. An accumulation or body of oil in sedimentary rock that yields petro- 

 leum on drilling. The oil occurs in the pores of the rock and is not a pool or 

 pond in the ordinary sense of these words. 



Outcrop. That part of a rock that appears at the surface. The^ppearance of a rock 

 at the surface or its projection abo\-e the soil. 



Paleontology. The study of the world's ancient life, either plant or animal, by 

 means of fossils. 



Peneplain. A region reduced almost to a plain by the long-continued normal ero- 

 sion of a land surface. It should be distinguished from a plain produced by the 

 attack of waves along a coast or the built-up flood plain of a river. 



Petrography. The description of rocks, especially of igneous and metamorphic 

 rocks with the aid of the microscope. 



Petrology. The study of rocks, especially of igneous and metamorphic rocks. 



Placer deposit. A m^ass of gravel, sand, or similar material resulting from the 

 crumbling and erosion of solid rocks and containing particles or nuggets of gold, 

 platinum, tin, or other valuable minerals. The valuable materials in placers have 

 been derived from rocks or veins by erosion. 



Playa (pronounced plah'ya). The shallow central basin of a desert plain, in which 

 water gathers after a rain and is evaporated. 



Porphyry. Any igneous rock in which certain crj^stal constituents are distinctly 

 visible in contrast with the finer-grained substance of the rock. 



Quartzite. A rock composed of sand grains cemented by silica into an extremely 

 hard ma?s. 



RhyoUte. A lava, usually of light color, corresponding in chemical composition to 

 granite. The same molten liquid tliat at great depth within the earth solidifies 

 - as gramte would, if it flowed out on the surface, cool more quickly and crystallize 

 less completely as rhyolite. 



Schist. A rock that by subjection to heat and pressure within the earth has under- 

 gone a change m 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 cer- 

 tain directions than m others. A schist has acrj^stalline grain roughly illustrated 

 by the grain of a piece of wood. 



^^^.^^l^^ .^!^^' formed by the accumulation of sediment in water 



deposits). The sediment may consist of 



^^iiqutftjurj ueposits; or xrom air 



rock frao-ments or nartirlps ^f 



animals 



flat 



Shale. 



product of chemical action or of evaporation (.alt, gypsnm, etc.); or of mixtures 

 o these materials. Some sedimentarj^ deposits (tufl.) are composed of fragments 

 blown from volcanoes and deposited on land or in water. A characteristic feature 

 of eedimentary deposits is a layered structure known as bedding or stratification. 

 Each layer is a bed or stratum. Sedimentary beds as deposited Ue flat or nearly 



consistin 



