FAULT (OF.. Fr. faute, Sp., Port., It. folio, flaw, from Lat. fallcre, to deceive. Gk. sphallein, to slip, Skt. pha I, to deceive, Lith. petit i, OHG. fallen, Ger. fallen, Icel. calla, AS. fea/htn, Eng. fall), or DISLOCATION. In geology, a dis placement of rocks along a plane of fracture. The plane of fracture may be inclined at any angle to the horizon, and the displacement may be vertical or horizontal, but usually both. The angle made by the fault plane with a vertical plane is called the 'bade' or 'slope' of the fault. The inclination of the fault plane with the horizontal plane is called the 'fault dip.' Where the displacement is partly vertical, the side on which the rocks lie at a higher level than that of their continuations across the fracture is called the 'upthrown side,' and the other is called the `downthrown side.' Also the side toward which the fault plane dips is called the 'hanging wall,' while the other side is called the 'foot-wall.' The `throw' of a fault is its vertical displacement. The horizontal displacement is sometimes called the 'heave.' Where the hanging-wall side of a fault is depressed with reference to the foot-wall side, the fault is said to be a normal or gravity fault. Where the hanging-wall side is thrust up over the foot-wall side, the fault is said to he a reversed or thrust fault. In a gravity fault
the dip of the fault plane is usually great; in a thrust fault, small. This displacement in normal faults is due to gravity, and causes the atTected bodies to occupy greater horizontal area. The displacement of a thrust or reversed fault originates in compression of the rock mass.
In general, faults are one of the manifestations of the deformation which rocks undergo in the outer parts of the earth's crust where they yield by fracture. (See CuusT OF THE EARTH.) Faults are accompanied by various phenomena such as breceiation, jointing (see •OINTS), and slicken siding, which are the common accompaniments of relief from pressure. While faults with marked throws are not very frequent. it is difficult to find a fracture in the earth's crust where some slight displacement of the parts has not occurred, for fractures develop in the relief of rocks from pressure, and relief cannot be obtained without at least a slight differential movement. In mountain masses the displacement by faulting may amount to hundreds, or even thousands, of feet. Sc)' MOUNTAIN; IIEOLOGY.