98 J. Barrett — Upper Devonian Delta of the 



tainous upland. The conglomerates lie mostly on the eastern 

 side in Connecticut, prevailing especially in the higher beds, 

 and are mostly of cobbles under a foot in diameter. They indi- 

 cate the repeated rejuvenation of hills not far distant from the 

 present eastern margin and appear to mark a zone of faulting 

 and uplift on the east during the progress of sedimentation to 

 the west. Thus, in a region of resistant metamorphic rocks, 

 erosion toward an early Triassic baselevel had become largely 

 completed. This does not mean, however, that a smooth plain 

 was necessarily, or even probably, developed across hard rocks. 

 Minor movements of the crust and the long time consumed by 

 the last stages of the erosion cycle may perpetuate a hilly relief 

 long after mountains have vanished. Irregularities in the 

 Triassic boundary in Massachusetts suggest that a mature 

 relief, measured by hundreds of feet, existed at the time of 

 burial, but this is a minor feature in comparison with the ero- 

 sion of thousands of feet which the metamorphic and crumpled 

 structures of the floor imply. The floor can be studied further 

 in Pennsylvania, on the southeast side of the Triassic. Here 

 it is tilted 15 to 20 degrees northwestward and erosion has cut 

 across both the Newark and the underlying formations. The 

 floor crosses pre-Cambrian gneiss, Cambrian quartzite, and 

 Cambro-Ordovician limestone. Although the margin is highly 

 irregular in detail, it is seen to intersect hard and soft forma- 

 tions without large response to the erosive resistance of the 

 formations. It is difficult to eliminate completely the influence 

 of faults and determine how much of the irregularity was due 

 to the original surface. In the Germantown quadrangle of 

 the Philadelphia folio* the irregularities in the margin of the 

 tilted floor suggest hills of gneiss rising to 1300 or perhaps 

 1700 feet above the pre-Newark valleys, but on the other hand 

 the contact crosses the broad anticline of pre-Cambrian in the 

 Phoenixville quadrangle for 18 miles without showing any 

 greater deflections than this from the limestone at each end. 

 The relief, therefore, was not due to the pre-Newark folding, 

 but was related to the hardness of rocks largely at least inde- 

 pendent of structure. 



It seems clear, then, that the folded structures of Permian 

 date had suffered profoundly from erosion even by the begin- 

 ning of Newark deposition, and the geology of the Newark 

 floor cannot be used as an argument to prove the absence 

 of the Upper Devonian from those formations which were 

 involved in the Permian folding. If they were high above 

 baselevel they would have suffered destruction before the be- 

 ginning of Newark time. Although such seems to be the 

 conclusion, it assists in the conception of the process of rapid 



* U. S. Geol. Survey. 



