258 DISLOCATIONS OF STRATA 



weight upon the beds that lie beneath the flexure, but increases 

 the weight upon the lines from which the arch springs. 



The problem regarding the way in which this great lateral 

 pressure was generated can best be considered in connection with 

 the study of mountain ranges. 



There is much independent evidence to show that folding is a 

 gradual process. The force exerted is enormous, but so is also 

 the resistance to be overcome, and a steady or oft-renewed com- 

 pression, acting upon strata under a great load of overlying 

 masses, will produce regular flexures, where a sudden compression, 

 however intense, could only shatter them. 



Thrust or reversed faults are likewise due to lateral compres- 

 sion, by which the rocks have been sheared and broken, and the 

 beds on one side of the plane of fracture have been thrust up 

 over those on the other. A plication or overturned fold may often 

 be traced into a thrust fault, in a way that shows the direction of 

 movement to have been the same in both fold and fracture. 

 Numerous experiments also show that lateral compression will 

 produce just such faults. A reduction of the overlying load, by 

 diminishing the plasticity of the rocks, will occasion shearing and 

 overthrusts, when, under a greater load, the same strata, exposed 

 to an equal force of compression, will simply flex and bend. As 

 we have seen (see Fig. 109), an anticlinal fold whose load has been 

 reduced by erosion, will, on renewed compression, fracture and 

 develop a thrust fault. 



Normal or gravity faults are due to tension, because, except in 

 the rare case of those with vertical hade, the faulted strata occupy 

 more space transversely than they did before faulting took, place. 

 It is not easy to explain how this tension has been generated. In 

 part, it seems to be due to a sinking of the downthrow side, 

 through the action of gravity, and, in part, to the application of a 

 compressing force acting at right angles to that which produced 

 the folds, thus raising the upthrow side into a very broad and 

 gentle arch, which is parallel to the plane of faulting. Faults of 

 this class are intimately connected with monoclinal folds, into which 

 they sometimes pass, as in the plateau region of Arizona and Utah. 



