386 The Mountain Axes near Cumberland Gap. [July, 
the formation of the Appalachian coal-field, so that the east and 
west section, from the Mississippi River to the sea across the 
Appalachian mountain system, gives us evidence of four distinet 
periods of elevation, the separation of which is recognizable on 
even a cursory inspection. I was not prepared, however, to find 
the additional evidence of the succession of elevations which has 
been given me by the study of the region lying between the 
Unaka Mountains of North Carolina and Central Kentucky. 
The work of the Kentucky Geological Survey in connection with 
the Harvard Summer School of Geology ranged during the last 
two summers over this area. As the results of this exploration 
must wait, it may be, some years before publication, I shall sum- 
marize some of the most important points that bear on this ques- 
tion. 
The geologist who is accustomed to the aspect of the Alle- 
ghanies in Pennsylvania will be struck with the change in the 
appearance of their continuation in East Tennessee. In place of 
the long-drawn symmetrical arches of the Pennsylvania section, 
we have here in East Tennessee a great irregular table-land 
crossed from north to south by narrow wall-like ridges, which 
have, in some cases, a length of over one hundred miles. A close 
study of the country shows that these ridges are in most cases 
the more or less retreated walls of fault lines, which have a sin- 
gular directness in their course and uniformity in the depth of 
their throw. 
The increase in the amount of faulting that took place in the 
formation of the Alleghany Mountains south of Pennsylvania 
becomes perceptible as we pass the Potomac River. In the 
mountainous regions of Virginia, along the waters of the south 
fork of that river, it begins to mark itself on the topography, and 
the change continually increases as we pass toward East Ten- 
nessee. Although still much in doubt as to the nature of the 
influences which have brought about this change, I venture to 
suggest the following explanation, which seems in a measure to 
satisfy the conditions of the problem. oes 
On looking at the sections exposed in Southwestern Virginia 
and Eastern Tennessee, it will be seen that there are two classes 
` of ridges found in this district: one formed by faults and the 
other by escarpments of the retreating crest of the anticlinals. It 
will be seen that the fault ridges have been formed on ti 
side of the anticlinal ridges, though there is but one considerab . 
ridge formed in this manner on the western side, while there 
