Joints.  At the surface, where their load is least, we find rocks universally broken into blocks of greater or less size by partings known as joints.  Under this name are included many division planes caused by cooling and drying; but it is now generally believed that the larger and more regular joints, especially those which run parallel to the dip and strike of the strata, are fractures due to up-and-down movements and foldings and twistings of the rocks.

Joints are used to great advantage in quarrying, and we have seen how they are utilized by the weather in breaking up rock masses, by rivers in widening their valleys, by the sea in driving back its cliffs, by glaciers in plucking their beds, and how they are enlarged in soluble rocks to form natural passageways for underground waters.  The ends of the parted strata match along both sides of joint planes; in. joints there has been little or no displacement of the broken rocks.

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.  The angle 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 and the foot wall.  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 sliekensides, 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.