620 STUDIES FOR STUDENTS 



may be deformed first by folding and afterwards by faulting. 

 Suppose a rock is deeply buried, but not so deeply as to cause 

 the superincumbent load to equal its ultimate strength. Sup- 

 pose the differential stress for a given stratum under these con- 

 ditions to surpass the elastic limit, but not to reach the ulti- 

 mate strength of the bed. It will then be deformed by folding, 

 but during the process shearing occurs on the limbs, and as a 

 result the bed is thinned, and finally the stress may surpass the 

 ultimate strength of the rock, which will then be fractured 

 and perhaps faulted. The same result may be reached if 

 before a stratum or formation is thinned the differential stress 

 increases so as to go beyond the ultimate strength of the rock. 

 It follows from this that deformation by folding followed by 

 faulting is normal for a considerable zone, for when the moun- 

 tain-making forces for a given region first begin their work it is 

 to be supposed that the differential stress is moderate. As the 

 stress increases in amount so as to exceed the elastic limit the 

 layers would begin to be folded, and fracture would occur as 

 soon as the differential stress reached the ultimate strength 

 of a given layer, provided the rock was not in the deep-seated 

 zone of flowage. 



It has been seen (p. 597) that accompanying thrust-faulting 

 fissility may develop parallel to the faults, and accompanying 

 overfolding cleavage may develop which dips in the same direc- 

 tion as the axial planes of the folds. In an area intermediate 

 between the zone of fracture and the zone of flowage, this being 

 at successive times under the conditions of flowage and of frac- 

 ture, there may be overfolding and cleavage combined with 

 thrust faults and fissility. In passing from a faulted to a folded 

 area, as has been noted, first there may be fissility along the 

 thrust faults, then the strata may be slightly overfolded and 

 tucked under along the faults, this undertucking becoming 

 more and more prominent and fissility at the same time being 

 replaced by cleavage, and finally we may have overfolds with 

 cleavage, with or without faulting. Each of the different phases 

 of the steps of change may occur on a large or small scale. In 



