STRUCTURAL DEPTH OF BASIN. 31 



North of Delaware Bay, and thence along the coast to Greenland, the 

 number of distinct coast erosion troughs increases and the evidence of their 

 great antiquity is very clear. The position of the Newark deposits in the 

 Connecticut Valley makes it evident that this region was an eroded basin 

 as far back as the Triassic period. The Narragansett Basin owes its 

 excavation to actions which antedate the Carboniferous. The Boston 

 Basin, and several others to the northward along the shores of Maine, may 

 be dated back to the Paleozoic age. Yet farther northward, wide valleys 

 of the coastal-plain type, though now deeply submerged, are indicated by 

 the reentrants of the Bay of Funcly, the Gulf of St. Lawrence, and prob- 

 ably by the great system of embayments of the Arctic realm, the Greenland 

 Straits and Hudson Bay, as well as by a host of lesser indentations, which 

 probably mark the seat of long-continued or, rather, frequently repeated 

 river action interrupted by periods of marine invasion. 



It will be observed from the statements made in this report that the 

 Narragansett Basin has at present an average structural depth of probably 

 not less than 7,000 feet and a maximum depth of 12,000 feet; that is to 

 say, the downfolded Carboniferous rocks and the beds which lie beneath 

 them attain, at the base of this incline, a position at least the last-named 

 distance below the present sea level. The question arises as to how much of 

 this geological depth is due to erosive work on the rocks of the area and 

 how much to actual depression preceding or connected with the folding of 

 the strata. If the basin originally had anything like its present depth, we 

 should have to suppose a very great change in the position of the coast 

 line. If, on the other hand, we may assume, as is done in this paper, that 

 the basin, as regards its geologic depth, is mainly the product of folding, 

 and that the movements are probably due in the main to the accumulations 

 of deposits, then the original depth of the basin may have been slight. 



The evidence seems to show that the coastal basins of the Atlantic 

 shore owe their depth to three more or less associated actions— to river 

 erosion, to downflexing and faulting associated with the accumulation of 

 strata during periods of subsidence, and to the massive swing of the conti- 

 nent in those large deformations such as have taken place in recent times, 

 with the consequent invasion of the sea into the valleys. Two of these 

 actions are local in their nature; the third involves continental or perhaps 

 wider conditions. 



