GLOSSAKY OF GEOLOGIC TERMS. 233 



Fault. A fracture in the earth's crust accompanied by movement of the rock on 

 one side of the break past that on the other. If the fracture is inclined and the 

 rock on one side appears to have elid do^\Ti the slope of the fracture the fault ia 

 termed a normal fault. If, on the other hand, the rock on one side appears to 

 have been shoved up the inclined plane of the break the fault is termed a reverse 

 fault. (See fig. 12, p. 100, and fig. 16, p. 1S8.) 



Pault block. A part of the earth's crust bounded ^vholly or in part by faults. 



Fault scarp. The cliff formed by a fault- Most fault scarps have been modified by 

 erosion since the^faultin^:. 



Fauna. The animals that inhabited the world or a certain region at a certain time. 

 Fissure- A crack, break, or fracture in the earth's crust or in a mass of rock. 



r 



Flood plain. The nearly level land that borders a stream and is subject to occasional 



overflow. Flood plains are built up by sediment- left by such overflows. 

 Flora. The assemblage of plants growing at a given time or in a given place. 

 Fold. A bend in rock layers or beds. Anticlines and synclines are the common 

 t>^es of folds. 



Pormation. A rock layer, or a series of continuously deposited layers grouped 

 together, regarded by the geologist as a unit for purposes of description and 

 mapping. A formation is usually named from some place where it is exposed 

 in its typical character. For example, Denver formation, Niobrara limestone. 



Fossil. The whole or any part of an animal or plant that has been preserved in 

 the rocks or the impression left by a plant or animal. This preservation is in- 

 variably accompanied by some change in substance, and in impressions the 

 original substance has all been removed, (See Pi. IV, p. 20, and PL XIX, p. 75.) 



Gneiss (pronounced nice). A rock resembling granite, but with its mineral con- 

 stituents so arranged as to give it a banded appearance. Most gnei^es are meta- 

 morphic rocks derived from granite or other igneous rocks. 



Granite. A crj-stalline igneous rock that has solidified slowly deep within the 

 earth. It consists chiefly of the minemls quartz, feldspar, and one or both of 

 the common kinds of mica, namely, black mica, or biotite, and white mica, or 

 musco\ite. The feldspar is the kind kno^^-n as orthoclase, and may be distin- 

 guished from quartz by its pale-reddish tint and its property of breaking with 

 flat shining surfaces (cleavage), for quartz breaks irregularly. The micas are 

 easily recognized by their cleavage into thin, flexible flakes and their bril- 

 liant luster. 



Horizon, In geology any distinctive plane traceable from place to pl^ce in different 

 exposures of strata and marking the same period of geologic time. A particular 

 horizon may be characterized by distinctive fossils. 



Igneous rocks. Rocks formed by the cooling and solidification of a hot liquid 

 material, known as magma, that has originated at unknown depths within the 

 earth. Those that have solidified beneath the surface are intrusive rocks, or, 

 if the cooling has taken place slowly at great depth, as plutonic intrusive or 

 plutdnic rocks. Those that have flowed out over the surface are known as effusive 

 rocks, extrusive ro^ks, or lavas. The term volcanic rocks includes not only lavas 

 but bombs, pumice, tuff, volcanic ash, and other fragmental materials or ejecta 

 thrown out from volcanoes. 



lithologic. Pertaining to lithology, or the study of rocks. (See also Petrology.) 

 Pertaining to rock character. 



Lode. An ore-bearing vein (see Vein); especially a broad or complex vein. 



I^oess (pronounced lurse with the r obscure). A fine homogeneous silt or loam 

 showing usually no division into layers and forming thick and extensi\'e 

 deposits in the Mississippi Valley and in China. It is generally regarded as in 

 part at least a deposit of wind-blown dust. 



