184 GUIDEBOOK OF THE WESTERN UNITED STATES. 
Loess (pronounced lurse with the r obscure). A fine homogeneous silt or loam — 
showing usually no division into layers and forming thick and extensive deposits 
in the Mississippi Valley and in China. It is generally regarded as in part at 
least a deposit of wind-blown dust. 
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 
streams in flowing across plains develop meanders. 
ism. 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 that have been so changed are termed 
metamorphic. Marble, for example, is metamorphosed limestone. 
Monzonite. An evyen-grained intrusive igneous rock intermediate in character 
between diorite and granite. It resembles granite. 
rain 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 appearance of a rock 
at the surface or its projection above the soil. (See Pl. IX, p. 57.) 
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 
erosion 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. 
trograph, 
stud k 
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, which have been derived from roc 
or veins. 
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 crystal constituents are distinctly 
visible im contrast with the finer-grained substance of the rock. 
Quartzite. A rock composed of sand grains cemented by silica into an extremely 
Rhyolite. A lava, usually of light color, corresponding in chemical composition to 
te. The same molten liquid that at great depth within the earth solidifies 
as granite would, if it flowed out on the surface, cool more quickly and crystallize 
less completely as rhyolite. 
ist. A rock that by subjection to heat and pressure within the earth has under- 
imentary rocks. Rocks formed by the accumulation of sediment in water 
(aqueous deposits) or from air (eolian deposits). The ‘sediment may consist of 
ries ments or particles of various sizes (conglomerate, sandstone, shale); of 
men 
blown from volcanoes and deposited on land or in water. A characteristic feature 
of sedimentary deposits is a layered structure known as bedding or stratifica- 
tion. Each layer is a bed or stratum. Sedimentary beds as deposited lie flat or 
nearly flat. (See Pl. VI, p. 52; Pl. XXVI, p. 114; Pl. XXXII, p. 126.) 
