514 GEOLOGY. 



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 beheved to affect rock struc- 

 tures 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 -j oints . 



Sandstone dikes. — Exceptionally, open joints are filled by the 

 intrusion of sedimentary material from beneath. Thus have arisen 

 the remarkable sandstone dikes ^ 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. 



Faults. — The beds on one side of a joint-plane or fissure are some- 

 times 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 posi- 

 tion, and hence fault-planes may 

 vary from verticality to approxi- 

 mate horizontality. The angle by 

 which the fault-plane departs from 

 Fig. 418.-Diagram of a normal fault. ^ vertical position is known as the 



hade (bac, Fig. 418). The vertical displacement (ac) is the throw and the 

 horizontal displacement (he) the heave. The heave and the throw are 

 to be distinguished from the displacement, which is the amount of 

 movement along the fault-plane (ah, 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 



^Diller. Bull. Geol. Soc. Am., Vol. I, pp. 441-442. Ibid, Hay, Vol. Ill, pp. 

 50-55; and Newsom, ibid Vol. XIV, pp. 227-268. 



