514 OF THE PHYSICAL STRUCTURE 



enormous mass of rocky material, thus forcibly pressed down and 

 firmly held there, would, we conceive, constitute a vast subterra- 

 nean barrier or dam, capable of arresting, in some degree, the 

 progress of the succeeding waves, and of protecting the region for 

 a moderate distance, towards the northwest, or the leeward side of 

 • the fault, from the undulations to which it would otherwise have 

 been exposed. In confirmation of this view, it may be stated, 

 that in tracing a line of dislocation toward either extremity, while 

 the extent of strata thrust down, as indicated by the amount of 

 the hiatus at the fault, is inferred to grow progressively less and 

 less, or, what is the same thing, the supposed subterranean dam, 

 presumed to diminish in depth, the region behind it, on the north- 

 west, becomes more and more undulated, until, when we pass 

 beyond the extremity of the fault, to where the normal form of the 

 flexm'e is restored, we find the strata to the northwest reared into 

 bold anticlinal and synclinal curves. Such is remarkably the fact 

 with the fault at the northwest base of the Peters's and East river 

 mountain, in Virginia, as well as with that which lies parallel 

 to, and southeast of, the Cumberland mountain ; and, in a word, 

 with all the faults and crushed axes of great length throughout 

 Virginia, Pennsylvania, and Tennessee. Even where two such 

 lines of dislocation occur, parallel to each other, at an interval of 

 not more than eight or ten miles, the central parts of the interven- 

 ing tract exhibit unusually little disturbance, notwithstanding 

 their proximity to the lines of violent disruption on each side. 



The assumed combination of the wave-like oscillation, and 

 horizontal or tangential movement, will explain, we believe, all 

 those general structural phenomena, which we have described as 

 characterizing our Appalachian chain in all its length and breadth, 

 and which obviously exist in many other mountain chains pos- 

 sessing numerous axes. It will account for all the varieties of 

 flexure, normal, inverted, or dislocated, which are any where ob- 

 servable in the chain, since a mere difference in the ratio of the 

 tangential to the undulatory movement, would produce every 

 grade and form of inflection we have had to record. 



The theory explains, moreover, the remarkable law of dimin- 

 ishing steepness in the flexures, as we cross the whole belt north- 



