1862.] 31 [Lesley. 



Mountain, beginning at Catskill on the Hudson, and ending in Ala- 

 bama. The northern portion of this plateau is drained eastward by 

 the branches of the Susquehanna, and westward by the branches of 

 the Alleghany and Monongahela Rivers. All the waters of Middle 

 Pennsylvania, Maryland, and Northern and Middle Virginia flow from 

 the foot of this escarpment towards the Atlantic, breaking successively 

 through the parallel Appalachian ridges of the subcarboniferous for- 

 mations. The waters of the Tennessee River head also at the eastern 

 foot of this escarpment, and flow along at its base for several hun- 

 dred miles southwestward, before they turn west at Chattanooga, and 

 break through its southern extremity, to make their great circuit 

 through Alabama, Western Tennessee, and Kentucky to the Ohio 

 River near its mouth. But in the middle of the region, namely, in 

 Southern Virginia, its normal drainage is reversed. The New River 

 heads in the Blue Range, crosses the Great Valley westward, breaks 

 into (not out of) the Appalachians, striking the escarpment in its 

 face, and flowing directly through and across it (as the Great Kanawha ) 

 through Western Virginia into the Ohio. 



The cause of this phenomenon is to be found in a change of struc- 

 ture at this line. Most of the valleys and mountains north of it as 

 far as New Jersey are unbroken anticlinals and synclinals. Most of 

 the valleys and mountains to the south of it, as flu- as Alabama, are 

 monoclinals, bounded by immense faults or downthrows. 



The Appalachian Mountains of Southern Virginia and Eastern 

 Tennessee are grouped in pairs by these faults. The country for 

 three or four hundred miles northeast and southwest, and from thirty 

 to forty miles from southeast to northwest, is fractured in parallel 

 strips from five to six miles wide. Each strip is tilted at such an 

 angle (dipping southeast) that at each fault the upper edge of one 

 strip (with its Carboniferous rocks) abuts against the bottom or Lower 

 Silurian edge of the strip next to it. As the Palaeozoic system, thus 

 revealed (on edge) between any two of these faults, contains two 

 massive sandrock formations. No. IV, Middle Silurian, and No. X, 

 Upper Devonian, there occur necessarily between each pair of fimlts 

 a pair of parallel mountains. The Palocozoic zone, therefore, included 

 between the Great Valley and the Backbone escarpment, is occupied 

 by as many pairs of parallel mountains as there are great parallel 

 faults; and as these foults range in straight lines at nearly equal dis- 

 tances from each other, these mountains run with remarkable unifor- 

 mity, side by side, for a hundred or two hundred miles, and are finally 

 cut off, either by short cross faults, or by slight angular changes in 

 the courses of the great faults. Thus we get an explanation of the 



