484 



ON THE PHYSICAL STRUCTURE 



character of the flexures of the strata. These flexures, unlike 

 tlie symmeti'ical curvature usually assigned to anticlinal and syn- 

 clinal axes, present, in almost every instance, a steeper or more 

 rapid arching on the northwest than southeast side of every con- 

 vex bend ; and, as a direct consequence, a steeper incurvation on 

 the southeast than the northwest side of every concave turn ; so 

 that, when viewed together, a series of these flexures has the form 

 of an obli(/i(c/ij undulated line, in which the apex of each upper 

 cm've lies in advance of the centre of the arch. On the south- 

 eastern side of the chain, where the curvature is most sudden, 

 and the flexures are most closely crowded, tiiey present a succes- 

 sion of alternat(!ly convex and concave folds, in each of which 

 the lines of greatest dip on the opposite sides of the axes, approach 

 to parallellism, and have a nearly uniform inclination of from 

 forty -five to sLxty degrees towards the southeast. This may be 

 expressed in other words, as a doubling' under or inversion of the 

 northwestern half of each anticlinal flexm-e. Crossing the moun- 

 tain chain from any point towards the northwest, the form of the 

 flexures changes, the close inclined plication of the rocks produc- 

 ing their uniformly southeastern dip gradually lessens, the folds 

 open out, and the northwestern side of each convex flexure, in- 

 stead of being abruptly doubled under and inverted, becomes 

 either vertical or dips steeply to the northwest. Advancing still 

 further in the same direction into the region occupied by the higher 

 formations of the Appalachian series, the arches and troughs grow 

 successively rounder and gentler, and the dips on the opposite 

 sides of each anticlinal axis, gradually diminish and approach 

 more and more to equality, until, in the great coal-field west of 

 the Allegheny mountain, they finally flatten down to an almost 

 absolute horizontality of the strata, at a distance of about one 

 hunch-od and fifty miles from the chain of the Blue ridge or South 

 mountain. 



These general features in the physical structure of the Appala- 

 chian region, will be best understood by consulting the Ideal sec- 

 tion, Plate XVI, intended to embrace the prevailing character of the 

 different portions of the chain from the Blue ridge to the western 

 coal-field. Along with this diagram, which embodies the gen- 



