1877.] The Mountain Axes near Cumberland Gap. 391 
that the relief of these mountain-building barriers has been 
found in two ways: First, by the folding of the earth into ridges 
and valleys, or synclinals and anticlinals ; second, by the forming 
of faults which are the product of the rupturing of the folds, and 
must be regarded as resulting from the failure of the lower lying 
rocks to follow and support the more superficial beds in their up- 
heaval. This failure may have been in part caused by the exceed- 
ing width of the upthrown ridge, which could not maintain itself 
until the deeper beds could adjust themselves to support it. 
When we consider the numerous cases in this district where 
thedrainage crosses faults of many thousand feet of throw, we are 
driven to the belief that whole geological periods have been re- 
quired for the movements involved in their formation. J am in- 
clined to believe, however, that while the greater part of these 
dislocations have been made slowly, some of them have been 
formed with a great suddenness, and attended by movements of 
extreme violence. Besides the faults traversed by streams whose 
courses have not been turned or interrupted by these gigantic dis- 
locations there are others which exhibit unequivocal evidence 
of violent movement in their formation. These evidences are 
various in their nature, but they are most conspicuous in the 
shattering of the walls on either side of the fault. The best in- 
stance of this sort of disturbance is found about the fault which 
passes through Cumberland Gap, and by the weakness it has 
given to the rocks has brought about the formation of this mount- 
ain pass. This remarkable fault is marked by the presence of a 
belt of rock fifty feet or more in width, which has been fractured 
into a breccia by the violence of the, movements which have at- 
tended its formation. The breccia is cemented by an infiltration 
of iron derived from the adjacent: carboniferous series of rocks. 
A careful study of this breccia has convinced me that the fault 
is the product of many successive movements, though each of 
them must have been attended by a certain beating of the walls 
against each other. This fault, it should be noticed, is trans- 
Verse to the direction of the great faults in this system of moun-_ 
tains, and is limited to the Cumberland synclinal, extending from 
a little beyond Cumberland Gap on the east to the gap in the 
Pine Mountain at Pineville. It differs also from the great parallel. 
northeast and southwest faults which we find at Clinch Mount- 
ain, Mound Hill, and other points, in the irregularity of its throw, 
which differs not only in amount but in direction in a curiously 
irregular way. I believe that it owes its formation to the com- 
