STRUCTURAL GEOLOGY 409 



if it " plunges," as it commonly does, a second set of tension-joints 

 roughly perpendicular to the first may be developed. If the uplift 

 is dome-shaped and sufficient to develop joints, they will radiate 

 from the center. It is true that joints affect regions where the 

 rocks have not been folded, and where they have been deformed 

 but little, but deformation to some extent is well-nigh universal. 



Shrinkage is a cause of certain minor tension-jointing. The 

 columnar structure of certain lavas, and the cracks developed in 

 mud when it dries, are examples. These causes, however, are not 

 believed to affect rock structures to great depths. 



Exceptionally, open joints are filled by the intrusion of sedi- 

 mentary material from beneath. Thus have arisen the remarkable 

 sandstone dikes 1 of the West, especially of California. 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 of by hydrostatic pressure. 



Faults. 2 The beds on one side of a joint-plane or fissure are 

 sometimes elevated or depressed relative to those on the opposite 

 side. Such a displacement is one type 

 of a fault (Figs. 33 and 34) . Joint-planes f 



and fault-planes may vary from verti- 

 cality to approximate horizontality. 

 The angle by which the fault-plane 

 departs from a vertical position is 



known as the hade (bac, Fig. 330). The 



..,,.! , \ . Fig. 330. Diagram of a 



vertical displacement (ac) is the normal fault. 



throw, and the horizontal displace- 

 ment (be) the heave. The heave and the throw are to be dis- 

 tinguished from the displacement, which is the amount of movement 

 along the fault-plane (ab, Fig. 330). 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 occasional fault-scarps of mountainous heights are 

 found, as along the east face of the Sierras and along many of the 



1 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. 



2 Chamberlin. Economic Geology, Vol. II, pp. 585 and 704. 



