390 The Mountain Axes near Cumberland Gap. [July, 
singularly low rate of fall; in several cases they run for miles 
without the least contact with their bed rocks, in fact with many 
feet of alluvial strata between their beds and the rock in which 
their troughs are excayated. I have been unable to explain 
this peculiarity save on the supposition that the district through 
which these streams course has been somewhat lowered by the 
movements which formed the Pine Mountain fault. If this be 
really the case, then we are compelled to suppose that the later 
movements of this dislocation — if the dislocation has indeed been, 
as I am disposed to believe, the product of a series of movements, 
— must have taken place after the drainage of this country had 
been entirely established, when each crest ran on its present line. 
- I am led to the opinion, all the evidence being taken into ac- 
count, only a part of which I can discuss here, that the escarp- 
ment of the Pine Mountain fault is now retreating from the line 
of breakage at the rate of not less than one foot in one hundred 
years. The rocks comprising the abrupt declivity are of a gen- 
erally perishable nature and wear out readily under the action 
of frost and rain. This rate of retreat would give an age of 
not. over five hundred thousand to one million and a half of 
years as the time that has elapsed since the formation of this 
fault. I am quite well satisfied that this estimate for the an- 
tiquity of the Pine Mountain fault is far within the truth, that 
it is in fact the result of disturbances which came in the time 
of the later Tertiaries. 3 
I hope to elaborate these observations on the conditions of the 
Alleghany system in the Memoirs of the Kentucky Geological 
Survey ; at present it is only possible to set forth the evidence 1n 
the briefest manner, with the special aim of calling the-attention 
of students of physical geology to the evidences of recent action 
in the mountain-building forces in this part of the Appalachian 
district. Iam confident that, more than any other mountain ac 
tion known to me, they tend to show that the strains which are re- 
lieved by mountain folds and faults are, in certain cases at least, 
continuous actions leading from time to time to movements that 
afford relief thereto. No one can study the structure of the section 
between the eastern face of the North Carolina mountains and the 
western side of the Cincinnati axis without being driven to the 
hypothesis that in a geological sense the mountains contained 
therein have been in a process of continuous formation since the 
beginning of the lowermost Cambrian deposits. Perhaps less 
distinctly shown, but it seems to me quite clear, is the evidence | 
