Appalachian Geosyncline. 97 



thing more than the time in which sedimentation has been 

 absent. 



For the Upper Devonian marginal sediments the problem is 

 somewhat different. A great mantle of terrestrial deposits 

 extended toward the mountains. The river grades which 

 determined whether erosion or deposition should take place 

 must have been subject often to minor disturbances of a 

 climatic or diastrophic nature. The disconformities produced 

 have been destroyed along with the marginal strata, but each 

 pulse of erosion on the margin of the plain must have been 

 recorded by a pulse of deposition of coarser waste toward the 

 center of the basin. Climatic and diastrophic movements dur- 

 ing and following the Upper Devonian would thus tend to 

 affect the margin and the center of the basin in different ways. 

 The relation between the two will, however, be treated more 

 fully in a later part of the paper. 



Degree of Destruction by the pre- Newark Cycle. 



The next related problem turning upon ancient baselevels 

 deals with the erosion of the early Mesozoic. Sedimentation 

 began about the middle of the Triassic in certain tracts of the 

 Appalachians ; giving rise to the red conglomerates, sandstones 

 and shales of the Newark group. These sediments rest upon 

 floors which had been intensely folded and metamorphosed 

 near the close of the Paleozoic, as shown by the Carboniferous 

 strata which are involved in the Maritime Provinces of Canada 

 and in Eastern New England. Further, near the Susquehanna 

 river in Pennsylvania the Triassic approaches within ten miles 

 of. intensely folded Upper Paleozoic rocks. Except in Nova 

 Scotia the Triassic nowhere, so far as the base is visible, rests 

 upon Devonian rocks. Does that mean that the Upper Devo- 

 nian never extended over the region now occupied by the 

 Triassic of the United States, or does it mean that sufficient 

 erosion could have taken place after the folding of the Permian 

 but before the deposition of the basal Newark of the mid-Tri- 

 assic to have removed thousands of feet of resistant Devonian 

 and Silurian formations ? 



Light on this question may be gained by an examination of 

 the floor of the Newark rocks. Davis long since called atten- 

 tion to the fact that the western contact in Connecticut is such 

 an old floor resurrected by Tertiary erosion. He notes that it 

 is approximately a tilted plain, since the outcrops extend in a 

 nearly straight line, except where broken by faults. It cuts 

 across greenstone and schist and granite without notable deflec- 

 tion. The basal beds of the Newark are moderately fine-grained 

 and are not the coarse agglomerates which skirt a rugged, moun- 



Am. Jour. Sci.— Fourth Series, Vol. XXXVII, No. 217.— January, 1914. 



