GLOSSARY OF GEOLOGIC TERMS. 135 



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

 from the Greek name of a nver in Asia IVIinor, \rhich hag a sinuous course. Moot 

 streams in flowing across plains develop meanders, 



Metamorphisru, 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- 

 sion into them of igneous rocks. Rocks so changed are termed metamorpluc. 



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

 between dioritc and granite. It resembles granite. 



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



Oil pool. An area or body of sedimentary rock that jdelds petroleum 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 appearance of a rock 

 at the surface or its projection above 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 studied with the aid of the microscope. 



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



Placer deposit. A mass 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 derived from rocks or veins by erodon. 



Playa (pronounced plah^^a). The shallow central basin of a d^ert plain, in which 

 water gathers after a rain and is evaporated, 



Pox-phyry, Any igneous rock in which certain crystal constituents are distinctly 

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



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



hard mass. 



E-hyolite. 



n chemical composition 

 within the earth solidif 



gramte. The same molten liquid tnat at great aeprn 



aa granite would, if it flowed out on the surface, cool more quickly and crj^stallize 



less completely as rhyolite. 

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

 gone 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 cer- 

 tain directions than in others. A schist has a crj^stalline grain roughly similar to 



the grain of a piece of wood. 

 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, g^ndstone, shale); of 

 the remains or products of animals or plants (certain limestones and coal); of the 

 product of chemical action or of evaporation (salt, gy^^um, 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 structiu-e known as bedding or stratification. 

 Eachlayerisabcdorstratum. Sedimentarybedsas deposited He flat or nearly flat. 



(See PI. XXVIII, p. 118.) 

 Shale. A rock consisting of hardened thin layers of fine mud. ^ ^ 



A ^^^v +v.a+ K^r «nhii:^riion to Drossure ^vithin the earth has acquired 



Slate. 



Qto thin plates. Tl 

 schist along its grain 



and 



