FAULTS 281 



Joints may remain closed, or may gape. They may be widened 

 liy Dilution, weathering, etc., and they may be filled by detritus 

 t'rom above, or by mineral matter deposited from solution (veins, 

 p. 286). Many rich ore- veins are developed along joint-planes. 



IP- 4.0- 



Joints have been referred to various causes, among which 

 tension, torsion, earthquakes, and shearing are the most important. 

 Mo>t of them may probably be referred to the tension or compres- 

 sion connected with crustal movements. 1 In the formation of a 

 simple fold, for example, tension-joints parallel with the fold will 

 be developed if tension goes beyond the limit of elasticity of the 

 rock involved. If the axis of a fold is not horizontal, that is, if it 

 pitches, 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 a slight extent is well-nigh universal. Shrinkage is 

 a cause of certain minor tension-jointing. The columnar structure 

 of lavas and sun cracks are examples. These causes, however, are 

 not believed to affect rock to great depths. 



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

 mentary material from beneath. Thus have arisen the remarkable 

 sandstone dikes 2 of the West, especially of California. Some such 

 dikes are 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. b /- 



Faults. 3 The beds on one = j= ~ 



side of a joint-plane or fissure 

 may be raised or sunk relative 4 mf-~ 

 to those on the opposite side. 

 Such a displacement is one type 



of a. fault (Figs. 2 7 I and 272). Fi ' -' A normal fault. 



Fault-planes vary from vertically to horizontality. The angle by 



1 Van Hise. Principles of North American Pre-Cambrian Geology; i6th 

 Ann. R.-pt., U. S. Gcol. Siirv., I't. I, pp. 668-672. 



- Diller. Bull. Geol. Spc Am., V,,l. I, pp. 441-442. Ibid., Hay, Vol. Ill, 

 PP- 5~55; and Ncwsom, ibid.. Vol. XIV, pp. 2 J 7-268. 



3 Various articles in Economic Geology, Vols. 1 and II; Cliumberlin, Fairchild, 

 Jaggar, Ransome, Reid, Spurr, and Willis. 



