=Faults.= In Figure 184 the rocks have been both broken and dislocated along the plane _ff´_. One side must have been moved up or down past the other. Such a dislocation is called a fault. The amount of the displacement, as measured by the vertical distance between the ends of a parted layer, is the _throw_ (_cd_). The angle (_ff´v_) which the fault plane makes with the vertical is the _hade_. In Figure 184 the right side has gone down relatively to the left; the right is the side of the downthrow, while the left is the side of the upthrow. Where the fault plane is not vertical the surfaces on the two sides may be distinguished as the _hanging wall_ (that on the right of Figure 184) and the _foot wall_ (that on the left of the same figure). Faults differ in throw from a fraction of an inch to many thousands of feet. =Slickensides.= If we examine the walls of a fault, we may find further evidence of movement in the fact that the surfaces are polished and grooved by the enormous friction which they have suffered as they have ground one upon the other. These appearances, called slickensides, have sometimes been mistaken for the results of glacial action. =Normal faults.= Faults are of two kinds,--normal faults and thrust faults. Normal faults, of which Figure 184 is an example, hade to the downthrow; the hanging wall has gone down. The total length of the strata has been increased by the displacement. It seems that the strata have been stretched and broken, and that the blocks have readjusted themselves under the action of gravity as they settled. =Thrust faults.= Thrust faults hade to the upthrow; the hanging wall has gone up. Clearly such faults, where the strata occupy less space than before, are due to lateral thrust. Folds and thrust faults are closely associated. Under lateral pressure strata may fold to a certain point and then tear apart and fault along the surface of least resistance. Under immense pressure strata also break by shear without folding. Thus, in Figure 185, the rigid earth block under lateral thrust has found it easier to break along the fault plane than to fold. Where such faults are nearly horizontal they are distinguished as _thrust planes_. [Illustration: Fig. 185. A Thrust Fault] In all thrust faults one mass has been pushed over another, so as to bring the underlying and older strata upon younger beds; and when the fault planes are nearly horizontal, and especially when the rocks have been broken into many slices which have slidden far one upon another, the true succession of strata is extremely hard to decipher. In the Selkirk Mountains of Canada the basement rocks of the region have been driven east for seven miles on a thrust plane, over rocks which originally lay thousands of feet above them.