200 GUIDEBOOK OF THE WESTERN UNITED STATES. 
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 slid down the slope of the fracture the fault is 
termed a normal fault. If, on the other hand, the rock on one side appears to 
have been shoved up the nes ag plane of the break, the ree is t d a reverse 
fault. (See fig. 20, p. 102; fig. 23, p. 112, and fig. 30, p. 1 
Fault block. A part of the earth’ 8 crust bounded wholly or in nls by faults. 
Fault scarp. Thecliffformed bya fault. Most fault scarps have been modified by 
erosion since the faulting. 
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 
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 overtiowes 
Flora. The assemblage of plants growing at a given time or in a given place. 
Fold. A bend in rock layers or beds. Anticlines = synclines are the common 
types of folds. (See fig. 28, p. 129, and fig. 31, p. 1 
Formation. A rock layer, or a series of continuously deposited layers grouped 
together, regarded by the geologist as a unit for purposes of description and 
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 haa been preserved in 
the rocks or the impression left by a plant or animal. This preservation is in- 
variably accompanied by change in substance, and from some impressions the 
original substance has all been removed. (See Pls. VI, A, p. 62; XI, B, p. 75.) 
Gneiss (pronounced nice). A rock resembling granite, but with its mineral con- 
ituents so arranged as to give it a banded appearance. Most gneisses are meta- 
morphic rocks derived from granite or other igneous rocks. 
Granite. A crystalline igneous rock that has solidified slowly deep within the 
earth. It consists chiefly of the minerals quartz, feldspar, and one or both of 
the common kinds of mica, namely, black mica, or biotite, and white mica, or 
muscovite. The feldspar is the kind known as orthoclase, and may be distin- 
Seep from quartz by its pale-reddish tint and its property of breaking with 
ining surfaces (cleavage), for quartz breaks irregularly. The micas are 
ache — by their cleavage into thin, flexible flakes and their bril- 
liant lus’ 
Horizon. a Sa any distinctive plane traceable from place to place in oe 
exposures of strata and marking the same period of geologic time. A partic 
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 known as intrusive 
sive or plutonic rocks. Those that have flowed out over the surface are known 
as effusive rocks, extrusive rocks, or lavas. The term volcanic rocks includes 
not only lavas but bombs, pumice, tuff, ee ash, and other fragmental 
materials or ejecta thrown out from volcan 
Lithologic. a to lithology, or the ae of rocks. (See also Petrology.) 
Pertaining to k character. 
dept in Git Mainsipg® Volley and in China. It is generally regarded as in 
lac se i ata 
