THE SANTA FE ROUTE, 183 
The rock fragments—soil, gravel, and silt—carried by a glacier. Drift in- 
cludes the perme material known as till and deposits made by streams flow- 
ing from a glacie 
Erosion. The wearing away of materials at the earth’s surface by the mechanical 
action of running water, waves, moving ice, or winds, which use rock fragments 
and grains as tools or abrasives. See Pl. XVII, p. 101; Pls. XXX-XXXIII, 
pp. 126-127.) Erosion is — by weathering. (See Weathering.) 
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 inclined plane of the break the fault is termed a reverse 
fault. (See fig. 14, p. 78; fig. 17, p. 92; fig. 28, p. 127.) 
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 ora 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 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 
types of folds. 
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. formation is cast named from some place where it is rR 
in its typical character. For example, Denver formation, N iobrara limesto 
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 This senate is ange 
= a by change i in substance, and fro or iginal 
e has all been rextioved. (See Pl. XXI, p. 107.) 
eiciee ‘casinca’ nice). A rock resembling granite, but with its mineral con- 
etitaer ts so arranged 3 a to give it a banded appearance. Most gneisses are meta- 
Granite. rystalline igneous ; rock that has solidified slowly deep within the earth. 
It me cies of the minerals quartz, feldspar, and one or both of the common 
s of mica, namely, black mica, or biotite, and white mica, or muscovite. 
The iawar is the kind known as orthoclase, and may be distinguished from 
surfaces (cleavage), for quartz breaks irregularly. The micas are easily recog- 
nized by their cleavage into thin, flexible flakes and their brilliant luster. 
Horizon. In geology any distinctive plane traceable irom place to place in different 
exposures of strata and marking the same period of geologic time. A particular 
Igneous rocks. Rocks formed by the cooling and solidification of a hot liquid ma- 
terial, known as magma, that has originated at unknown depths wi within the earth. 
if the cooling has taken place slowly at great depth, as plutonic intrusive or 
preg tas egies Those that have flowed out over the surface are ‘known as effusive 
rocks, ex ve rocks, or lavas. The te 
but aoe pumice, tuff, volcanic ash, sail other ee materials or ejecta 
wn out from volcanoes 
Lithologic. Pertaining to Helslogy, 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. 
