1877.] The Mountain Axes near Cumberland Gap. 387 
are three or more on the eastern side. After a good deal of 
consideration of these peculiar features, I have come to the con- 
clusion that this change of the structure on the southward ex- 
tension of the Alleghanies can best be accounted for by assuming 
the following conditions: First. That in place of the relatively 
narrow ridges of the Pennsylvania district, the uplifts which 
occurred here took the shape of one or more very broad anti- 
clinals having a transverse width of sixty miles or more. Second. 
That each of these anticlinal axes was fractured by faults along 
several lines for its whole length, the result being to tumble the 
fissured strata over each other, leaving only the central part of 
the anticlinal still complete. Third. That the more massive the 
Blue Ridge to the eastward becomes, the more intense do we find 
this faulting action, on the east showing some relation between 
this faulting and the mass of the old mountains. I have long 
been of the opinion that the faulting in any mountain region be- 
Comes greater as the anticlinals widen or tend to take on some 
of the characters of the Alpine “ massifs.” It is not difficult to 
imagine a reason for this general occurrence of faults in broad 
folds of mountain masses ; a small fold may have some sustaining 
power to its arch, and can await the gradual movement of strata 
to fill up its suddenly formed cavities. A broad fold will neces- 
sarily be weaker ; the creeping of the strata into positions fitted 
to sustain the uplifted ridge may not be quick enough to keep 
furrows from forming, as they have formed in this Tennessee 
district. ; 
It is not so easy to perceive a reason for the greater width of 
the anticlinals in this part of the Appalachian chain. There 
are, however, good mechanical reasons why the width of the 
ridges and furrows which make up a mountain range should have 
a width proportionate to the depth of the strata involved in the 
Movement. An illustrative experiment showing the principle that 
determines this is easily made by taking a number of sheets of 
eavy paper and compressing them from the sides. The more sheets 
we affect by the pressure, the broader the resulting folds will be. 
In any region where successive mountain upheavals have taken 
Place, as in the Alps, there is often evidence going to show that. 
the lateral force operated at first to disturb the more superficial 
beds, and then in succession the deeper ones became affected. It 
is this, I believe, that gives us the Swiss massife plan of 
Mountain-building. I conceive that in the Alpine region there 
Was first a set of folds after the essential plan of the Jura Mount- 
