Round cibout Ashemlle. 295 



face of the valley, but they all plunge by a more or less abrupt 

 cascade into the main rivers. It is thus evident that the tributa- 

 ries cannot keep pace with the rivers in channel-cutting, and the 

 latter will continue to sink below the surface of general degrada- 

 tion until their diminished fall reduces their rate of corrasion 

 below that of the confluent streams. 



If froili topographic forms we turn to consider the materials, 

 the rocks, of which they are composed, we shall find a general 

 rule of relation between relative elevation and rock-hardness. 

 Thus the great valley of East Tennessee has a general surface 

 3000 feet below the mean height of the Unakas : it is an area of 

 easily soluble, often soft, calcareous rocks, while the mountains, 

 consist of the most insoluble, the hardest, silicious rocks. East 

 of the Unakas the surface is again lower, including the irregular 

 divide, the Blue Ridge ; here also, the feldspathic gneisses and 

 mica schists are, relatively speaking, easily soluble, and non- 

 coherent. What is thus broadly true is true in detail, also where 

 a more silicious limestone or a sandstone bed occurs in the valley 

 it forms a greater or less elevation above the surface of the soft 

 rocks ; where a more soluble, less coherent stratum crops out in 

 the mountain mass, a hollow, a cove, corresponds to it. Of 

 valley ridges, Clinch mountain is the most conspicuous example ; 

 of mountain hollows the French Broad valley at Hot Springs, or 

 Tuckaleechee Cove beneath the Great Smoky mountain, is a fair 

 illustration. 



But impassive rock-hardness, mere ability to resist, is not 

 adequate to raise mountains, nor is rock-softness an active agent 

 in the formation of valleys. The passive attitude of the rocks 

 implies a force, that is resisted, and the very terms in which that 

 attitude is expressed suggest the agent which applies the force. 

 Hardness, coherence, insolubility, — these are terms suggestive of 

 resistance to a force applied to wear away, to dissolve, as flowing 

 water wears by virtue of the sediment it carries and as perco- 

 lating waters take the soluble constituent of rocks into solution. 

 And it is by the slow mechanical and chemical action of water 

 that not only canons are carved but even mountain ranges 

 reduced to gentle slopes. 



If we designate this process by the word " degradation," it 

 follows from the relation of resistance to elevation in the region 

 under discussion that we may say : The Appalachians are moun- 

 tains of differential degradation ; that is, heights remain where 



