Fig. 417.—Sandstone dike. Northern California. (Diller, U. S. Geol. Surv.)
A minor cause of tension-jointing is shrinkage, due (1) to cooling, as in the development of the columnar structure of certain lavas, and (2) to dessication, as shown by the cracks developed in mud when it dries. These causes, however, are not believed to affect rock structures to any considerable depth. Torsional joints and joints due to earthquake vibrations appear to be special phases of tension-joints.
Two or more sets of joints may also be produced by compression, the number being dependent on the complexity of the folding. Many compression-joints correspond in direction with planes of shearing. They are often associated with minor faulting and with slaty cleavage.
Tension-joints appear to be much more widely distributed than compression-joints.
Sandstone dikes.—Exceptionally, open joints are filled by the intrusion of sedimentary material from beneath. Thus have arisen the remarkable sandstone dikes[218] of the West, especially of California ([Fig. 417]). Such dikes are sometimes several miles (nine at least) in length. The sand of these dikes was forced up from beneath either by earthquake movements or by hydrostatic pressure.
Fig. 418.—Diagram of a normal fault.
Faults.—The beds on one side of a joint-plane or fissure are sometimes elevated or depressed relative to those on the opposite side, and the displacement is known as a fault (Figs. 418 and 419). The joint-planes may have any position, and hence fault-planes may vary from verticality to approximate horizontality. The angle by which the fault-plane departs from a vertical position is known as the hade (bac, [Fig. 418]). The vertical displacement (ac) is the throw and the horizontal displacement (bc) the heave. The heave and the throw are to be distinguished from the displacement, which is the amount of movement along the fault-plane (ab, [Fig. 418]).
The cliff above the edge of the downthrow side is a fault-scarp. In many, probably in most cases, the scarp has been destroyed, or at any rate greatly obscured by erosion; but occasionally fault-scarps of mountainous heights, as along the east face of the Sierras and along many of the basin ranges of Utah, Nevada, etc., are found though much modified by erosion ([Fig. 419]).
Faults sometimes arise from over-intense folding ([Fig. 420]). A deformation which at one point results merely in a bending of the beds, may occasion a fault at another. Faults may pass into folds either vertically (Figs. [421] and [422]) or horizontally ([Fig. 423]). In such cases, thickening and thinning, and stretching and shortening of the beds is often involved (see Figs. [421] and [422]). Faults are often due to the greater settling of the beds on one side of a fissure than on the other, without special disposition to fold.