228 



STRUCTURE COMMON TO ALL ROCKS. 



Cause. — The cause of great fissures is evidently always movements, 

 either by foldings or by liftings of the earth's crust. In either case 

 there would be formed a parallel system of fissures in the direction of 

 the folds, and therefore at right angles to the direction of the folding 

 or lifting force. Fissures are usually thus found in systems parallel 

 among themselves, and to the axes of mountain-chains. Through such 

 fissures igneous rooks in a fused condition are often forced, forming 

 dikes and overflowing sheets. Besides the principal fissures just ex- 

 plained, Hopkins has shown that, in the case of the formation of mount- 

 ains, there would be formed also other smaller fissures at right angles 

 to these. 



Nearly always the walls on the two sides of a fissure do not corre- 

 spond with each other, but one side has been pushed up higher or 

 dropped down lower than the other. Such a displacement is called a 

 fault, a slip, or dislocation. This may occur in fissures in any kind of 

 rock, but is most marked and most easily distinguished in stratified 

 rocks. When the strata are sufficiently flexible to admit it, they are 

 bent instead of broken, and a monoclinal fold is formed instead of a 

 fault (Fig. 195). When the fissure is filled at the moment of its forma- 



Fig. 195.— Section of Nutria-Fold, New Mexico (after Gilbert). 



tion with fused matter from beneath, it is called a dike. When it is not 

 filled at the moment of its formation with igneous injection, but sloivly 

 afterward with other matter, and by a different process, it is called a 

 vein. Dikes we have already discussed (p. 209) ; veins we will discuss 

 later ; we are concerned here only with faults. 



Faults. — In faults the extent of vertical displacement varies from a 

 few inches to hundreds or even thousands of feet. In the Appalachian 

 chain there occur faults in which the vertical dislocation is 5,000 to 



Fig. 196.— Fault in Southwest Virginia: a, Silurian; d, carboniferous (after Lesley). 



