^338 A. KEITH OUTLIXES OF APPALACHIAN STRUCTURE 



morphism increase in number and strength toward the southeast. If 

 the j^lateau mass were underthrust against the faulted mass, the reverse 

 should be true, and the greatest intensity of all these structures should 

 be along the southeast margin of the plateaus, where the advancing mass 

 under the plateaus was thrust against the belt of sharp folds. In this 

 situation, as has been described, the Paleozoic sea-fioor slopes upward 

 toward the west. The scene was therefore set for a maximum localiza- 

 tion of strain and therefore of its effects along this line. Southeastward 

 the effects of the pressure should be distributed and diminished, if 

 Tmderthrusting were the real process. As has already been stated, there 

 is a sharp change in the amount of deformation in that situation, but 

 "there is by no means a maximum there. Metamorphism, an evidence of 

 maximum pressure, is not exhibited there at all. It is not noticeable 

 for 30 or more miles to the southeast, and in increases definitely aAvay 

 from the plateaus. Xor do the folds and faults decrease eastward. The 

 few exceptions to this rule are due to local changes in the stratigraphic 

 -column and hardly form one per cent of the whole. Therefore, on the 

 principle that where the maximum pressure is applied there the maxi- 

 mum results should be found and should diminish at a distance, the case 

 is plainly against the theory of underthrust on the northwest and for the 

 theory of overthrust from the southeast. 



HOW THE DEFORMING FORCE WAS TRAXSMITTED 



Deformation of the strata has proceeded to various degrees of comple- 

 tion in the Appalachians. In the slightly deformed beds it is possible 

 to assign a variety of modes of operation of the folding. In the deforma- 

 tion of moderate intensity, such as the open folds of the Great Valley, it 

 is obvious that the massive strata, like dolomite and quartzite, have car- 

 ried the l^runt of the load and transmitted the strain from one area to 

 :another, while the weak shales and thin limestones have failed. In the 

 areas of maximum deformation at the southeast of the range only those 

 rocks of the greatest strength have preserved their form to any extent, 

 •and in many places even these have been sheared and mashed by the 

 strain until they can scarcely be recognized, as, for instance, a fine mus- 

 •covite schist derived from the granite. In all of the phenomena one 

 fact stands out — that is, that the weak beds have had little part in trans- 

 mitting the deforming force and that the rigid beds transmitted the 

 most of it mitil they were overcome in succession by the eastward in- 

 crease of the forces. Each mass, whatever its rigidity, had to be sufficient 

 unto itself or it failed. This was on account of the lack of cohesion 



