of miners, is the sudden interruption of the
continuity of strata in the same plane, accompanied by a crack or
fissure, varying in width from a mere line to several feet, which is
generally filled with broken stone, clay, &c.

[Illustration: Fig. 99.]

The strata, _a_, _b_, _c_, &c., must at one time have been
continuous; but a fracture having taken place at the fault F, either
by the upheaving of the portion A, or the sinking of the portion B,
the strata were so displaced that the bed _a_ in B is many feet
lower than the same bed _a_ in the portion A.

FAUNA. The various kinds of animals peculiar to a country constitute
its FAUNA, as the various kinds of plants constitute its FLORA. The
term is derived from the FAUNI, or rural deities, in Roman
Mythology.

FELSPAR. A simple mineral, which, next to quartz, constitutes the
chief material of rocks. The white angular portions in granite are
felspar. This mineral always contains some alkali in its
composition. In _common felspar_ the alkali is potash; in another
variety, called Albite or Cleavlandite, it is soda. Glassy felspar
is a term applied when the crystals have a considerable degree of
transparency. _Compact_ felspar is a name of more vague
signification. The substance so called appears to contain both
potash and soda.

FELSPATHIC. Of or belonging to felspar.

FERRUGINOUS. Any thing containing iron. _Etym._, _ferrum_, iron.

FISSILE, easily cleft, dividing readily into an indefinite number of
parallel laminae, like slates.

FLOETZ ROCKS. A German term applied to the secondary strata by the
geologists of that country, because these rocks were supposed to
occur most frequently in flat horizontal beds. _Etym._, _flotz_, a
layer or stratum.

FLORA. The various kinds of trees and plants found in any country
constitute the FLORA of that country in the language of botanists.

FLUVIATILE. Belonging to a river. _Etym._, _fluvius_, a river.

FORAMINIFERA. A name given by D'Orbigny to a family of microscopic
shells. Their different chambers are united by a small perforation
or _foramen_. Recent observation has shown that some at least are
not Cephalopoda, as D'Orbigny supposed.

FORMATION. A group, whether of alluvial deposits, sedimentary
strata, or igneous rocks, referred to a common origin or period.

FOSSIL. All minerals