250 ROLLIN T. CHAMBERLIN 



folded belt suggests that this deep insetting may constitute the mode 

 by which the moving shell anchors itself in the less moving mass below 

 and thus determines where the movement shall cease and the folding 

 shall take place. This function I think is usually assigned to some 

 specially stable portion of the earth-body. The plunge is here 

 correlated with thick sedimentation and this may be an agency in 

 inducing the plunge. If other cases shall support this suggestion it 

 may offer a new view of the well-known relation between thick 

 sedimentation and mountain-folding. 



In Fig. 6 the line BC does not pass through the point where the 

 strata were being shaped preparatory to faulting. This may be due 

 to inaccuracies in the field work and in the reconstructed section. 

 But Daubree's experiment showed that there may be fracturing along 

 several closely parallel lines near the edge of the moved block and 

 that these may be broadly included in the accommodation zone. 

 The angle ABC at the apex of the block is not far from a right angle. 

 The fact that it is slightly less than 90° may be due perhaps to the 

 fact that the triangular block has been laterally compressed, which 

 would lessen the original angle at the apex, but such an explanation 

 is not required as the variations of the angle natural to the case more 

 than cover the departure from a right angle. 



The likelihood of a zone of accommodation between the folded 

 shell and the less movable interior where deformation by flowage is 

 presumed to be the prevalent type, has been brought out in Chamberlin 

 and Salisbury's Geology J Near Harrisburg the moderately thin 

 movable shell seems to have been so sharply crumpled that the adjust- 

 ment between it and the solid support beneath would seem to have 

 been accompanied by much shearing. But west of the Blue Ridge, 

 where the folding was less intense and the compression was distributed 

 through a much thicker segment, the adjustment between the more 

 movable portion above and the less movable portion below may have 

 been accomplished mainly by distributive shear. The flexures on the 

 surface presumably pass downward into the zone of quasi-fiowage 

 where they accommodated themselves by distributive deformation. 



Turning back from these details to the general problem of estimat- 

 ing the thickness of the folded shell, it may be recalled that the 

 calculation commences with three dimensions obtained from the field 



' Vol. 11, p. 130. 



