246 ROLLIN T. CHAMBERLIN 



NATURE OF THE DEFORMED BLOCK 



Fig. 6 expresses diagrammatically the variable thickness of the 

 shell beneath the different sections of the folded tract, as developed 

 by this method of analysis. The separate sections are each drawn 

 to true scale as independent blocks, and constitute a series of steps. 

 As the diagram rather strikingly shows, the folded shell is thinnest at 

 the eastern end of the section and thence thickens step by step west- 

 ward up to a certain turning-point, beyond which it shallows even 

 more quickly. The regularity of these descending steps, when first 

 worked out, came as a distinct surprise as it was not anticipated 

 from the simple inspection of the reconstructed cross-section. It 

 seems to be a feature of much significance and appears to give a con- 

 crete picture of how the deformation occurred. 



In order to approximate more closely what may be supposed to 

 have been the actual nature of the deformation, the broken lines 

 AB and BC are drawn as substitutes for the artificial steps. These 

 pass through the middle points of the bottom lines of each block with 

 the exception of Section 2, the deepest segment, and hence the resulting 

 sectional area of each segment remains essentially as it was in the 

 rectangular block. Segment 2 is the apex, and to keep its area the 

 same as the original block, the triangle FBG is constructed so as 

 to equal in area the sum of the triangles DEF and GHI. At the 

 same time Segments i and 3 retain their original areas. 



Taken together the stepping-blocks from which the lines AB 

 and BC have been derived are very suggestive. The thrust which 

 produced these great mountain-flexures came presumably from the 

 direction of the Atlantic Ocean. The near-by eastern side of the 

 folded tract suffered more intense crumpling than the more remote 

 western portion, the sharpest folding lying in the tract east of the 

 present Blue Ridge. West of this the surface folds slowly die out 

 with, however, one great fold at the west end. Simultaneously with 

 the diminishing intensity of the folds, the thickness of the folded shell 

 increases. Apparently the thrust from the Atlantic Ocean affected 

 at first a moderately thin crust of five or six miles, or perhaps even 

 less, which it squeezed intensely. From this thin, intensely com- 

 pressed strip the lateral thrust was transmitted to the region lying 

 immediately west; but instead of being communicated simply to the 



