Ch. XXV.] CAEBONIFEKOUS GKOUP. 389 



standing the phenomena now alhided to, although I must guard him 

 against supposing that it is a true section. A great number of details 

 have of necessity been omitted, and the scale of heights and horizontal 

 distances are unavoidably falsified. 



Starting from the shores of the Atlantic, on the eastern side of the 

 Continent, we first come to a low region (a b), which was called the 

 alluvial plain by the first geographers. It is occupied by tertiary and 

 cretaceous strata, before described (pp. 180, 231, and 254), which are 

 nearly horizontal. The next belt, from b to c, consists of granitic rocks 

 (hypogene), chietl}'- gneiss and mica-schist, covered occasionally with 

 unconformable red sandstone, No. 4 (New Red or Trias?), remarkable 

 for its footprints (see p. 346). Sometimes, also, this sandstone rests 

 on the edges of the disturbed paleozoic 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 charac- 

 terized by hills of moderate height, contrasting strongly, in Ir^eir 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 difi'erent 

 colors, running in a IST. E. and S. W. direction, in the same way as the 

 lias, chalk, and other secondary formations in the middle and eastern 

 half of England. 



The narrow and parallel zones of the Appalachians here mentioned, 

 consist of strata, folded into a succession of convex and concave flexures, 

 subsequently laid open by denudation. The component rocks are of 

 great thickness, all referable to the Silurian, Devonian, and Carboniferous 

 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, geologically considered, from Vermont to 

 Alabama, being more than 1000 miles long, from 50 to 150 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 Pennsylvania, 

 Prof. W. B. Rogers and his brother Prof. H. D. Rogers, for tlie import- 

 ant discovery of a clue to the general law of structure prevailing through- 

 out 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 bend- 

 ing 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 posi- 

 tion. By reference to the section (fig. 505), it will be seen that on the 



