.370 REPORT OF NATIONAL MUSEUM, 1904. 



standpoint, the most important and far-reaching- conclusions, were those 

 h. d. andw.B. P 11 * forward by W. B. and H. D. Rogers in a paper 

 Appalachian rea d before the Association of Geologists and Natural- 



structure, 1842. ists in 1842 ^ ent i t i ec i Qn the physical structure of the 



Appalachian chain as exemplifying the laws which have regulated the 

 elevation of great mountain chains generally. They called attention 

 to the fact that the chain consists of a broad zone of very numerous 

 ridges of nearly equal height, characterized by their great length, 

 narrowness, steepness of slope, evenness of summit, and a remarkable 

 parallelism. They divided the chain on topographical and structural 

 grounds into nine distinct divisions, consisting of alternating straight 

 and curved portions. They noted the remarkable preponderance of 

 southeastern dips throughout the entire length, although the general 

 trend of the chain was northeast and southwest. This was particu- 

 larly characteristic of the portion along the southeastern or most dis- 

 turbed portion of the belt, but toward the northwest dips in the 

 opposite direction became less steep and more numerous. 



They accounted for the phenomena of these dips by assuming them 

 to be due to a scries of unsym metrical flexures, presenting in most 

 instances steeper or more rapid arching on the northwest than south- 

 east side of every convex bend; and, as a direct consequence, a steeper 

 incurvation on the southeast than northwest side of every concave 

 turn; so that, when viewed together, a series of these flexures has the 

 form of an obliquely undulating line, in which the apex of each upper 

 curve lies in advance of the center of the arch. On the southeast- 

 ern side of the chain, where the curvature is most sudden, and the 

 flexures are most closely crowded, they present a succession of 

 alternately convex and concave folds, in each of which the lines of 

 greatest dip on the opposite sides of the axes approach to parallelism 

 and have a nearly uniform inclination of from 45° to 60° toward 

 the southeast. This they described as a doubling -under or inversion 

 of the northwestern half of each anticlinal flexure. Crossing the 

 mountain chain from any point toward the northwest, the}^ noted 

 that the form of the flexures changes; the close inclined plication of 

 the rocks producing their uniformly southeastern dip gradually les- 

 sens, the folds open out, and the northwestern side of each convex 

 flexure, instead of being abruptly doubled under and inverted, becomes 

 either vertical or dips steepby to the northwest. Advancing still 

 farther in the same direction into the region occupied Iry the higher 

 formations of the Appalachian series, the arches and troughs grow 

 successively rounder and gentler, and the dips on the opposite side of 

 each anticlinal axis gradually diminish and approach more and more 

 to equality, until, in the great coal field west of the Allegheny Moun- 

 tains, they finally flatten down to an almost absolutely horizontally of 



