604 STUDIES FOR STUDENTS 



been kneaded again and again by the orogenic forces ; cleavage^ 

 fissility, and band banding may have developed in different 

 directions ; earlier structures may have been destroyed by later 

 transformations ; until it is no longer possible to determine the 

 position of original bedding. 



APPLICATION TO CERTAIN REGIONS. 



In many mountainous regions in which there has been pro- 

 found erosion, illustrations of nearly all of the foregoing princi- 

 ples may be found. Attention may be directed to one or two of 

 the more important. 



It has already been pointed out (pp. 337-338) that in the 

 Appalachian and New England crystalline areas the main direc- 

 tion of active stress was probably from the southeast toward the 

 northwest. At any rate, the couple composed of force and 

 resistance was such as to make the higher strata move toward 

 northwest as compared with the lower strata, or to make the 

 lower strata move to the southeast as compared with the upper 

 strata. As a consequence of this, the folds of that area have 

 axial planes which have a very general tendency to dip to the 

 southeast. If the force be supposed to have been directed 

 toward the northwest, and to have been equal for different depths 

 throughout the thickness of the rocks now exposed at the sur- 

 face, the cleavage which developed in the normal planes would 

 dip to the southeast. This would have been due to two causes : 

 First, the direction of normal pressure, compounded of thrust 

 and of gravity, would have been northwest and downward, which 

 would, therefore, have given a southeasterly dip to the cleavage. 

 Also, because of increasing resistance and probably lessening 

 thrust with increasing depth, the force would have caused the 

 higher strata to have moved differentially over the lower ones. 

 There would, therefore, have been a shearing motion, the higher 

 strata moving upward and northwestward as compared with the 

 lower. As a result of this shearino" and of shortenino;, the old 

 and new mineral particles would lie with their longer diameters 

 in southeasterly-dipping planes and give a cleavage in that direc- 



