202 DOUGLAS WILSON JOHNSON 



conclusion, added another paragraph in which they said that, while 

 they formerly regarded the argument as conclusive, further study of 

 other similar divides had led them to modify this opinion. They 

 cited the divides between the Potomac and James, and between the 

 James and Roanoke basins, as cases similar to the one in question, 

 but where there was no reason, so far as known, for supposing that a 

 large stream ever existed. Evidently, then, this one of the arguments 

 is only of negative value; it is permissive, but in no wise conclusive. 

 That in this particular case the divide really presents features which 

 make it difficult to conceive how a large stream could ever have 

 flowed across it, is pointed out in subsequent pages. 



Evidence from the volume of material eroded and deposited. — The 

 second line of evidence developed by Hayes and Campbell is as 

 follows: If we can ascertain the quantity of sediments carried out 

 by the Alabama River system during Tertiary times, and can also 

 ascertain the quantity of material eroded from the basins of the 

 Coosa-Alabama system and the upper Tennessee system during 

 Tertiary times, we can then compare the two results and find out 

 whether the amount of sediments deposited is balanced by the amount 

 of material eroded from the Coosa-Alabama basin alone, or whether 

 it is only balanced when we add to it the material eroded from the 

 upper Tennessee basin also. Lines are drawn midway between the 

 Alabama and the Chattahoochee on the east, and between the Ala- 

 bama and Tombigbee on the west, and all the Tertiary sediments 

 between the two lines regarded as the material carried out by the 

 Alabama system in Tertiary times. The amount is estimated at 

 2,340 cubic miles. Turning now to the amount of material eroded 

 from the basin of the present Alabama system during Tertiary times, 

 a careful estimate places the amount at 622 cubic miles. It is evident 

 from these figures that some other area must have contributed to 

 the formation of the vast amount of sediments laid down opposite 

 the old mouth of the Alabama. If we add to the material eroded 

 from the basin of the present Alabama system the material eroded 

 from the basin of the upper Tennessee system, it is found that the 

 total material eroded from both basins, during Tertiary times, is 

 about 2,500 cubic miles. This agrees closely enough with the 2,340 

 cubic miles of Tertiary sediments found opposite the old mouth of 



