134 GUIDEBOOK OF THE WESTERN UNITED STATES. 



Favdt, 



crust 



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 inclined plane of the break, the fault is termed a reverse 

 fault. (See PI. XXXIII, B, p. 127; fig. 12, p. 90; fig. 15, p. 128.) 



Fault block. A part of the earth's crust bounded wholly or in part by faults. 



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

 erosion since the faulting. 



Fauna. The animals that inhabited the world or a certain region a.t 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 overflows. (See 

 PI. XXIX, A, p. 119.) 



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 sjTicUnes are the common 

 types of folds. (See fig. 15, p. 128.) 



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 

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

 its tj^ical character. For example, Denver formation, Niobrara limestone. 



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



left on rock by a plant or animal. Preservation ia 

 invariably accompanied by some change In substance, and from some impressions 

 the original substance has all been removed. (See PL XXV, p. 92.) 



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

 stituents so arranged as to give it a banded appearance. Most gneisses are meta- 

 morphlc rocks derived from granite or other igneous rocks. 



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

 earth. It consists chieflv of ihf 



pression 



nu 



mxiscovite. 



. mica, or Diotite, and wnite mica 



iwn as orthnrlasR and mn\r "Ha <1i« 



guishod 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 brilliant luster. 



Horizon. In geology any distinctive plane traceable from place to place in different 

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

 horizon may be characterized by distinctive fos.dls. 



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

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

 earth. Those that have solidified beneath the surface are known as intrusive 



or plutonic rocks. Those that have flowed out o" 

 sive rocks, extrusive rocks, or lavas. The term 

 lavas but bombs, pumice, tuff, volcanic ash, ai 

 ejecta thrown out from volcanoes. 

 Litholcgic. Pertaining to lithology, or the study 

 Pertaining; to rock character. 



intrusive 



iwn 



Petrology 



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

 Loess (pronounced lurse with the r obscure). A fine homogeneous silt or loam show- 

 ing usually no_ division into layers and forming thick and extensive deposits in 

 the Mississippi \ alley and in China. It is generally regarded as in part at least 

 a deposit of wind-blown dust. 



