498 CARBONIFEROUS GROUP. [Ch. XXV. 



stone rests on the edges of the disturbed palaeozoic rocks (as seen in 

 the section). The region (b c), sometimes called the "Atlantic 

 Slope," corresponds nearly in average width with the low and flat 

 plain (a b), and is characterized by hills of moderate height, con- 

 trasting strongly, in their rounded shape and altitude, with the long, 

 steep, and lofty parallel ridges of the Alleghany Mountains. The out- 

 crop of the strata in these ridges, like the two belts of hypogene and 

 newer rocks (a b and b c), above alluded to, when laid down on a 

 geological map, exhibit long stripes of different colors, running in a 

 N. E. and S. W. direction, in the same way as the lias, chalk, and 

 other secondary formations in the middle and eastern half of Eng- 

 land. 



The narrow and parallel zones of the Appalachians, here mentioned, 

 consist of strata, folded into a succession of convex and concave flex- 

 ures, subsequently laid open by denudation. The component rocks 

 are of great thickness, all referable to the Silurian, Devonian, and Car- 

 boniferous formations. There is no principal or central axis, as in the 

 Pyrenees and many other chains — no nucleus to which all the minor 

 ridges conform ; but the chain consists of many nearly equal and 

 parallel foldings, having what is termed an anticlinal and synclinal 

 arrangement (see above, p. 48). This system of hills extends, geologi- 

 cally considered, from Vermont to Alabama, being more than 1000 

 miles long, from 50 to 1.50 miles broad, and varying in height from 

 2000 to 6000 feet. Sometimes the whole assemblage of ridges runs 

 perfectly straight for a distance of more than 50 miles, after which all 

 of them wheel round altogether, and take a new direction, at an angle 

 of 20 or 30 degrees to the first. 



We are indebted to the state surveyors of Virginia and Pennsyl- 

 vania, Prof. W. B. Rogers and his brother Prof. H. D. Rogers, for the 

 important discovery of a clue to the general law of structure prevail- 

 ing throughout this range of mountains, which, however simple it may 

 appear when once made out and clearly explained, might long have 

 been overlooked, amidst so great a mass of complicated details. It 

 appears that the bending and fracture of the beds is* greatest on the 

 southeastern or Atlantic side of the chain, and the strata become less 

 and less disturbed as we go westward, until at length they regain their 

 original or horizontal position. By reference to the section (fig. 552), 

 it will be seen that on the eastern side, or in the ridges and troughs 

 nearest the Atlantic, southeastern dips predominate, in consequence 

 of the beds having been folded back upon themselves as in i, those on 

 the northwestern side of each arch having been inverted. The next 

 set of arches (such as Jc) are more open, each having its western side 

 steepest ; the next (I) open out still more widely, the next (m) still 

 more, and this continues until we arrive at the low and level part of 

 the Appalachian coal field (d e). 



In nature or in a true section, the number of bendings or parallel 

 folds is so much greater that they could not be expressed in a diagram 



