1871.] Ill [Shaler. 



able salients at these points are easily accounted for ; the first is 

 obviously the product of the Mississippi, and the second the result of 

 the action of the reef-building corals (as has been shown by the 

 researches of Agassiz and others), aided, it may be, by the action of 

 the mud of the Mississippi. It is evident that this projection at Cape 

 Hatteras is due to neither of the causes which have produced these 

 more southern reliefs. It is not like the Louisiana salient, the product 

 of the detritus deposited at a river's mouth, for the rivers emptying at 

 this point are small and carry but little amount of detritus ; besides, it 

 will be evident from the details of the geological structure of this 

 region that the shore is waning at the mouths of the rivers rather 

 than gaining in extent. It is not to be regarded as the product of 

 corals, like the great monument of organic life, the promontory of 

 Florida. Professor Agassiz has, it is true, suggested that the sand 

 reefs which fringe the Cape may be merely ancient coral reefs now 

 serving as a basis of sand accumulations. It seems, indeed, natural to 

 assume that these reefs, which continue to the northward, are the 

 outlines of the coral barriers, or possibly have some relation thereto. 

 Careful study, however, has shown that the changes which occur in this 

 barrier are of such a character as to forbid our accepting this view. 

 Passes open and close in the barrier, cutting apparently to its base 

 which seems to be a dense clay. The truth is, these ridges of sand 

 in the form of barrier reefs are common along the whole coast where 

 moving sands occur, from Montauk Point to Mexico. They seem to 

 be the necessary product of tidal action on any shore where there 

 are large amounts of materials in a condition to be moved by the 

 currents produced by the tides. 



I have elsewhere endeavored to account for the presence of the 

 enormous masses of detrital material which constitutes the broad 

 reefs of Hatteras, and have shown that it is likely that the excavation 

 of the great bays of the Chesapeake and Delaware was accomplished 

 during the geological period just passing away; and that the exca- 

 vating agents were the streams of ice which at that time poured down 

 the valleys which debouch at the heads of these great inlets, just as 

 the great alpine glaciers have in times past dug out the basins 

 of the Swiss lakes. I have tried to show that the material so exca- 

 vated, or at least a good part of it, drifted to the southward of these 

 bays and went to form the great masses of reef material which make 

 up the Hatteras bars and those which form the Eastern border of 

 the broad waters of Eastern Virginia. 



