422 GEOLOGY. 



that subaerial erosion was not taking place at a rapid rate, and that 

 the principal formation of the epoch, as far as our continental area 

 is concerned, was extracted from the sea-water. 



The Helderbergian series is 300 to 400 feet thick in eastern New 

 York, and 350 to 600 feet in Pennsylvania. It thins to the west, 

 being but a few feet thick in Ohio, and 100 feet or less in western Ten- 

 nessee. In the vicinity of Gaspe, there is a continuous formation 

 of limestone some 2000 feet thick, a part of which is of Helderbergian 

 age. 



The Oriskany. — The Oriskany formation is best known in the 

 Appalachian region, but beds of the same age have a wide but undeter- 

 mined distribution in the eastern part of the Mississippi basin, and 

 in the St. Lawrence basin in the southern part of Ontario. Beds con- 

 temporaneous with the eastern Oriskany are more widespread than 

 the eastern Helderberg formation. In the region where best known, 

 the Oriskany is made up chiefly of coarse-grained sandstone. The 

 vicinity of Cumberland, Md., may be regarded as the center of 

 the Oriskany of the mountain belt. From this locality, it extends 

 both northeast and southwest, but thins in both directions and loses 

 its most distinctive faunal characteristics. To the westward, the 

 faunal characteristics of the Appalachian belt are found as far as 

 Cayuga in Ontario, indicating that the barrier between the Ap- 

 palachian trough and the interior — a barrier which seems to have 

 been effective since the Utica epoch — was removed during this 

 epoch, allowing the life of the former region to migrate into the latter. 

 In the southerly part of the Mississippi basin (western Tennessee) 

 the Oriskany fauna also appears, though the formation in which it 

 occurs (Camden chert) has no lithological likeness to the Oriskany 

 of the east. In general, the beds of the interior, which are perhaps 

 contemporaneous with the Oriskany of the Appalachian belt, are not 

 sharply differentiated from the overlying beds, either lithologically 

 or paleontologically. 



As developed in the Appalachian belt, the Oriskany is a thin forma- 

 tion ranging from a score of feet in New York to several hundred feet 

 in Maryland and Virginia. 



In the northeast (Gaspe, New Brunswick, and Nova Scotia) the 

 Helderberg formation has not been clearly differentiated from over- 

 lying parts of the Devonian system, but beds containing the Oriskany 



