STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 447 



variable to account for the unequal distribution of sediments which I 

 have ascribed to local differential movements. 



The long downwarps prevailing during Paleozoic times in the Appa- 

 lachian Valley may have originated chiefly in continental spreading, and 

 the same process probably contributed as much as any other cause to 

 their gradual deepening in the course of their depositional loading. 

 Indeed, this seaward creep probably aided greatly in the subsequent 

 stronger folding of these Appalachian troughs — if not by actual outward 

 movement, then at least by its opposition to the oceanic inland thrust. 

 The usual attitude of the Appalachian folds, with their steep westerly 

 and less steep easterly dips, is not opposed to this view, since the force 

 of the outward creep of the continent would expend itself chiefly on 

 the lower part of the fold, while the emergent tendency of the suboceanic 

 spread would affect the upper part of the fold rather than the lower. 



Inland transmission of suboceanic thrusts, — Assuming (1) the peri- 

 odicity of diastrophism, (2) the competence of "suboceanic spreading" 

 to cause landward thrusting of marginal areas of continents, and 

 (3) that seaward "continental creep" is an important factor in the 

 causation of deformative movements, certain conceptions relating to the 

 inland transmission of suboceanic thrusts are suggested. In the first 

 place it seems reasonable to believe that stresses accumulated in the 

 oceanic area until they were strong enough to overcome the resistance 

 offered by continental creep. Before reaching this point, local and 

 perhaps general deepening of the oceanic basins occurred, with the prob- 

 able if not inevitable result of sea withdrawal from the continents. Obvi- . 

 ously, the land movement would manifest itself first in the marginal 

 areas l)cneath which a tangential thrust or shear-])! ane (see page 440) 

 would sooner or later be established. Previous to this, as is suggested, 

 for instance, by the great withdrawal of interior continental seas at the 

 close of the Trenton, the continent as a whole may have responded to 

 the deep-seated landward pressure or flowage by general median ele- 

 vation. As a rule, however, much of this interior doming seems to have 

 been deferred till after the development of the tangential thrust-plane 

 and the decided warping and folding which occurred in the belt at its 

 emergence (see north-south tilting and note on faulting, pages 407 and 

 433). 



Despite the relief afforded by the development of this tangential thrust- 

 plane, horizontal movement in the matter beneath it probably continued. 

 Theoretically, this lower zone should at first have responded a little to 

 the stress of continental spread, but soon the drag in the deeply buried 

 shear zone may have stopped and then reversed the seaward movement 



