PRIxXCIPLES OF STRATIGKAPHIC CORRELATIONS 57B 



rocks that in early Paleozoic ages were deeply buried; and presumably 

 much the same was true of the marginal strip before it was covered by 

 the mantle of Mesozoic and Cenozoic rocks. In consequence of repeated 

 elevation and erosion the Paleozoic marine deposits, which were here and 

 there laid doAvn on the broad and then unstable surface of Appalachia, 

 were rather generally removed. Perhaps, indeed, the present remnants 

 of these deposits, as, for instance, the Ordovician slates on the Virginia 

 Piedmont, owe their preservation largety to burial beneath overthrust 

 masses which, following subsequent general elevation and westward migra- 

 tion of the belt of folding (see pages 435 to 442), were later removed hy 

 erosion so as to again expose the crushed slates. 



Assuming (1) that Appalachia was at one time traversed by longi- 

 tudinal and transverse folds similar to those indicated in the Appalachian 

 A^alley and (2) that in their essential features crustal folds are perma- 

 nent structures, it follows that the arched areas were less likely to retain 

 the occasional sediments than the downwarped areas. Moreover, the low 

 parts of the anticlinal ridges which were submerged at such times proba- 

 bly formed connecting straits in which deposition must have been limited 

 and possibly prohibited by current efficiency. Hence it is only in the old 

 basins that any of the depositional record of the Atlantic connections of 

 Appalachian Valley formations could possibly be preserved ; and if there 

 is any truth in the suggested hypothesis of inland migration of the belt 

 of active folding, such remnants must be very rare. According to this 

 hypothesis the belt of active folding began on the eastern border of the 

 land and migrated farther and farther westward across Appalachia until 

 some age in the Mesozoic era, since which time it has ridden as a whole 

 H'ithout folding on deep-seated sheer planes. Rising thus as on an in- 

 clined plane, the surface of Appalachia, especially over its western half, 

 has suffered long continued and often vigorous degradation. That the 

 whole surface of this old land-mass has otherwise remained relatively 

 stable since the transgressing belt of folding reached its western margin 

 is shown by the slight deformation of the marine deposits laid on it dur- 

 ing and after the Triassic. Differential tilting, gentle warping, and nor- 

 mal faulting continued, but folding ceased long ago. 



For the reasons given it is not surprising that the depositional records 

 of former Atlantic connections seem to have been almost obliterated. 

 Still, though rare, there are at least two areas that retain fairly satisfac- 

 tory renmants. One of these is in the Skunnemunk Mountain in south- 

 ern N"ew York and tlie adjacent Green Pond Mountain in T^ew Jersey, 

 which region contains Devonian deposits that suggest communication with 

 the Atlantic and the Oulf of ^fexico. Measured according to the strike 



