DEFORM A TION OF ROCKS 



619 



fractures, with scarcclv any bowing of the layers of the rocks 

 along the shear planes (see Figs. 4 and 6 on p. 468, and pp. 596- 

 598). In passing to the greater depths the load is greater, and 

 the la3'ers, instead of all having the full movements of the clean- 

 cut thrust faults, adjacent to the fault planes may be found to 

 be in sharp overfolds in opposite directions upon opposite sides 



Fig. 16. — Fold passing into fault. 



of the faults (Fig. 16). Where the load is still greater these 

 folds are of increased importance. Under still greater load the 

 rocks may be first bent into an overfold, with little faulting, and 

 finally at a greater depth the deformation may occur altogether 

 by overfolding. It is therefore clear that in the same mountain 

 mass there may be all gradations between clean-cut thrust 

 faults and overfolds without faults. The transition may be 

 longitudinal, as in the case of the Appalachians, where thrust 

 faults which occur in the extreme southeast are gradually 

 replaced by overfolds to the northwest. Also the transition 

 ma}^ be transverse. In the latter case, if erosion cuts the strata 

 to different depths after such a region was deformed, the over- 

 folds may be found in the central parts of the mountain mass, 

 the transition phases upon the immediate areas, and the 

 thrust faults without overfolds upon the outer flanks of the 

 mountains. 



Rocks at a certain depth, and therefore under a definite load, 



