APPALACHIAN BASELEVELS 273 



close together the hight relations of their summits would no more sug- 

 gest the existence of an ancient elevated baselevel than do those of the 

 Swiss Alps. It is rather the long, even skylines of their ridges than their 

 likeness in hight that so distinctly suggests their origin in an elevated 

 plain. 



Pkoblem of Appalachian Baskleveling 



There are two sections of the Alleghany district which are especially 

 noteworthy in this inquiry : One of them is the area about Cumberland 

 gap, especially in the section adjacent to the headwaters of Cumberland 

 river; the other in the drainage of the upper James river near Buffalo 

 gap. In the first named area is an extensive tract of country where the 

 ridges attain a hight of about 3,000 feet, rising to a considerable elevation 

 above the neighboring crests. It is evidently a case where the superior 

 hight is to be accounted for by the fact that we have there the node 

 whence a number of streams radiate, flowing outwardly with a relatively 

 slight fall, so that their cutting power is not great. In the second case, 

 that near Bufi"alo gap, the excessive hight of Elliots knob is evidently 

 due to its peculiar structure, which, in the absence of a strong attack by 

 streams, has failed to go down as rapidly as the neighboring crests. 

 These are but two instances out of many which go to show that the 

 wide discrepancies in hight which exist in this region can be explained 

 by variations in thie conditions of down-cutting rather than by any ref- 

 erence to the problem of the original plane in which the erosion began. 



We may fitly ask certain questions of those who hold to the hypothesis 

 of an ancient elevated baselevel on which the erosion of the Alleghany 

 section began. The first of these concerns the probable hight of the 

 ancient surface above the present summits of the region. It is evident 

 that it must have been very much above the plane of the existing crests. 

 There are probably few geologists Avho would reckon the period in which 

 this part of the Appalachian has been subjected to erosion since the 

 Mesozoic period, when the supposed elevation took jdace, at less tlian 

 ten million j^earSjOr the rate of down-wearing at less than 1 foot in 2,000 

 years. On this computation the original surface must have been 5,000 

 feet above the summits of the existing crests. Even if Ave halve this esti- 

 mate we still leave the supposed plane very far up in the air — so far up 

 indeed, that it does not seem reasonable to assume its sometime exist- 

 ence to account for the slight measure of uniformity which exists in the 

 hights of the existing crests. 



The evident difiiculties of the view that holds for a baselevel control 

 of the Alleghany ridges are increased when we come to consider the con- 



