REPORT ON THE GKOLOGY OF NORTH AMERICA. 17 



similar boulders in the neighbourhood of Columbia, on the Sus- 

 quehanna, containing several species oiProducta and Terebra- 

 tula, which could only have come from a like region within the 

 mountains of Pennsylvania, a distance perhaps of fifty miles at 

 least. Drake, in his Picture of Cincinnati, mentions large 

 masses of granite in that part of Ohio, resting upon the ordinary 

 finer diluvium. The nearest granite on the north is at least one 

 hundred leagues distant ; while no primary rocks occur south or 

 east within even a much greater limit. We are reminded here 

 of the great detached blocks which strew the plains of northern 

 Europe, and the explanation suggested that they have been car- 

 ried there by floating upon ice. They occur, promiscuously dis- 

 persed over a great extent of country in Ohio, Kentucky, and 

 Indiana, and are in no way connected with the present river 

 valleys. 



I may mention as an interesting fact, corroborating the opi- 

 nion of the northerly origin of the current here advanced, that 

 Mr. Conrad, who has explored the State of Alabama, was never 

 once able to perceive a boulder upon its surface. 



Besides the fossiliferous deposits of very recent date, descx'ibed 

 by Mr. Conrad, around the Gulf of Mexico, many of our rivers 

 adjacent to the sea present extensive beds of shells, of another 

 class, but probably referrible to the same origin and the same 

 period of elevation. They consist of the common Ostrea virgi- 

 nica, almost exclusively, with a very few of the recent univalves 

 of the coast, all of these being shells peculiar to the bays and 

 estuaries of the rivers, and the shallow sounds on the inner side 

 of the Sea Islands and shoals along the coast. The position in 

 which these beds of shells are invariably seen is upon the low 

 level plains adjacent to the tide creeks of our rivers, where they 

 appear to have dwelt in colonies in the sheltered bays at a time 

 when these plains were at a small depth beneath the water, and 

 to have been lifted with them by, perhaps, the last shock which 

 has changed the level of the coast. These shells, in a sub-fossil 

 state, occur in Cumberland County, New Jersey, on the bank of 

 Stow Creek, at Egg Harbour, on the Severn, at Euston, in 

 Maryland ; again, upon the York river in Virginia, and indeed 

 upon many others of the southern rivers. They occur at the 

 mouth of the Potomac, resting upon the beds of marine shells, 

 which were originally described in the Journal of the Academy 

 of Natural Sciences by Mr. Conrad, and considered by him as 

 referrible to the newest of our fossiliferous formations. In the 

 same locality these beds of fossil Ostrea virginica are seen to 

 be covered by the diluvium, so that there can be no question of 

 their origin having been during the latest §tage, as it were, of 



X834. C 



