OF THE APPALACHIAN CHAIN. 507 



Increasing Interval between the Axes as we advance 

 Northwestward. 



It is an interesting general fact, that the space between the 

 axes, or, more properly, the amplitude of the undulations, increases 

 as we cross the chain northwestward. This is represented in the 

 ideal Section, and is equally apparent in the actual Sections 

 which accompany it, being strikingly visible in that (Section B) 

 intended to illustrate the sti'ucture of the Juniata resrion. Althousfh 

 distinctly noticeable in the northwestern side of the belt, the gra- 

 dation prevails equally in the middle and southeastern tracts, 

 though in the latter the numerovis minor flexures, with the inter- 

 ference of groups of different dimensions, prevents our at first 

 perceiving the law in all its simplicity and exactness. Towards 

 the southeastern side of the chain, the flexures become so nume- 

 rous, and are so often folded or inverted, as, in most cases, to ren- 

 der the comparison of their distances impracticable. Yet, even 

 in this quarter, the general truth appears, in the diminished space 

 between the foldings, as we cross the Great Valley, southeast- 

 ward. Taldng in the whole breadth of the chain, the prevalence 

 of the rule is obvious, no matter by what Section we cross. 



PART IT. 



Theory of the Flexure and Elevation of the Strata, 

 founded on the preceding Phenomena, — combined undu- 



LATORY and TANGENTIAL CHARACTER OF THE MOVEMENT. 



That the movement which prockiced the permanent flexures 

 was compounded of a wave-like oscillation, and a tangential or 

 horizontal pressure, both propagated northwestward across the 

 disturbed belt, is plainly indicated by the oblique character of 

 nearly all the anticUnal and syrtcUnal curves, both those which 

 are closely folded, and those which are obtuse. This obhque in- 

 flection of the strata will, we confidently believe, be found to pre- 

 vail as the regular form of all anticlinal axes, in every part of the 



