928 



A TREATISE ON METAMOEPH1SM. 



transport the superjacent material of the zone of fracture with it, and thus 

 carry forward the upper crust of the earth toward the mountain mass and 

 allow the segregation of all of the superficial deformation in one belt. 



It is to be noted that in the theoretical case illustrated by the diagram 

 there is no crustal shortening of the belt in the zone of fracture or of the 

 material of the zone of flowage. The material has merely changed its 

 position with reference to the surface of the earth 



From the foregoing it is not to be supposed that monoclinal cleavage 

 has generally been produced without any plications or crustal shortenings. 

 Indeed, in areas in which monoclinal cleavages are prevalent there may 

 be numerous folds, as, for instance, in the New England region and in the 

 Blue Rido-e area. No doubt in cases where the deformation is consider- 



Fig. 25. — Diagrams illustrating the manner in which deformation in the zone of flowage may concentrate crustal shorten- 

 ing in the zone of fracture. 



able there are all gradations, from very little or no crustal deformation of 

 the superjacent rocks to that of the complex kind which is characteristic 

 of the local zones of intense deformation. And even if the suggestion 

 be true in general, it is believed that in any instance there is likely to be 

 more or less of monoclinal folding in connection with the monoclinal cleav- 

 age, and consequently more or less of crustal shortening. However, it is 

 thought that the hypothesis affords a possible explanation of the concentra- 

 tion of the greater part of the crustal shortening in local zones. 



The question now arises whether, in the regions of the earth in which 

 the past zone of flowage is now at the surface, monoclinal cleavage exists. 

 That such cleavage does exist for some extensive regions is well known. 

 For instance, the crystalline area of the Appalachian system from Maine to 

 Georgia shows a dominant easterly dipping cleavage. 



