246 J. Barrett — Upper Devonian Delta of the 



The rivers do not appear to have been great continental 

 water-ways. Such are far apart in their mouths, correspond- 

 ing to the size of their drainage basins. The bulk of the sedi- 

 ment of such rivers is mud and silt. They corrade their way 

 across mountains at a low grade and would not be capable of 

 building sloping piedmont plains. Furthermore the coarse- 

 ness of the quartzite bowlders in the Skunnemunk conglomerate 

 is such that rivers probably did not carry them from the moun- 

 tains more than 75 or 100 miles. For the sources of erosion 

 we must look east and southeast from the geosyncline, toward 

 a parallel and not far distant mountainous land. 



Let the attention be turned next from the sediments to the 

 terranes beyond which could have supplied them. The 

 geologic maps show that within the zone once covered by the 

 Devonian piedmont gravels, Cambrian and Ordovician rocks 

 still remain over considerable areas, and evidence has been 

 given that the pre-Cambrian ridges of gneiss east of the 

 Schuylkill river were mostly mantled by younger formations 

 up to Jurassic times. West of the Schuylkill the prevailing 

 formations within the original Upper Devonian limits are 

 Ordovician rocks. Even, therefore, if previous arguments had 

 not indicated that this zone was a region of deposition, it could 

 not be regarded as a field of supply for an appreciable portion 

 of the sediment. 



Passing next to a zone to the east of the last and beginning 

 at the north, it is to be noted that the pre-Cambrian rocks of 

 the Green Mountains are of comparatively small areas. As 

 far east as the valley of the Connecticut and as far south as 

 New York city, large areas of Cambrian and Ordovician 

 schists remain. South and east of New York City the ocean 

 and the deposits of the Coastal Plain largely conceal the older 

 structures ; but, beginning at Trenton on the Delaware River, 

 the ancient rocks reappear between the Triassic and Com- 

 manche overlaps, constituting an exposure of gneisses and 

 granites widening southwards through Pennsylvania into 

 the Southern Appalachians. Near the Delaware state line, 

 however, a few outliers of Cambrian quartzite and limestone 

 still remain, and immediately west are large areas of metamor- 

 phosed Ordovician rocks. This exposed belt, about 25 miles 

 wide, certainly suffered very considerable erosion in early 

 Triassic and again in Jurassic times. There is no evidence 

 that it was covered by Upper Devonian sediments, but neither 

 could it have suffered much from Upper Devonian erosion. 

 Thus, by a process of exclusion, for the zone of mountains 

 which supplied this waste we must look farther to the eastward, 

 at least as far as eastern Connecticut and the region now over- 

 lain by the strata of the Coastal Plain. This could have been 



