110 Hayes and Campbell—Appalachian Geomorphology. 
It should be remarked that while the writers formerly regarded 
the character of the divides between these drainage basins as 
conclusive evidence that large streams flowed across them until 
the close of the Tertiary period of baseleveling, they have recently _ 
found reasons for modifying this conclusion. A study of the 
divides between drainage basins throughout the Appalachian 
valley from Pennsylvania southward shows that most of them 
are quite perfectly reduced to the altitude of the Tertiary pene- 
plain in adjacent basins, although not generally so broadly cut 
as the one in question. There is no reason, so far as known, for 
supposing that the divides between the Potomac and James or 
the James and Roanoke basins have shifted during the Tertiary 
cycle, yet they are nearly as inconspicuous as those between the 
Tennessee and Coosa. On the other hand, the divide between 
the New and Holston basins has the form of a narrow col, 
such as would be expected to characterize all long-maintained 
divides. | 
Kvidence from the Volume of Material eroded and deposited—The 
second line of evidence bearing on the date at which the Appa- 
lachian drainage was diverted to its present westward course is 
derived from a comparison of the volumes of Tertiary erosion 
and Tertiary sediments. It is comparatively easy to compute 
the volume of the material which was removed by the rivers 
during the Tertiary cycle, when the vertical distance between 
the previously existing peneplain and the one developed during 
the Tertiary cycle is known, together with their lateral extent ; 
also a tolerably safe estimate may be made of the volume of 
sediments deposited by each of the rivers during the Tertiary 
cycle. If the drainage during the whole of the cycle was essen- 
tially as it is at present, then the volume of sediments which 
would naturally be deposited by the present streams and the 
volume of the material eroded by those streams should show a 
practical agreement. The formations laid down during the Ter- 
tiary cycle are regarded as including (1) the Ripley—sands and 
sandy clays overlying the Rotten limestone and marking the 
uplift which terminated the preceding cycle; (2) Lignitie; (8) 
Buhrstone ; (4) Claiborne; (5) White limestone *—a series de- 
creasing in coarseness and increasing in amount of calcareous 
*The Tertiary and Cretaceous Strata of the Tuscaloosa, Tombigbee and 
Alabama Rivers, by Eugene A. Smith and Lawrence C. Johnson: U. S. 
Geological Survey, Washington, Bull. 43, 1887, 189 pp., 21 pls. 
