390 APPALACHIAN CHAIN. [Ch. XXV. 



eastern side, or in the ridges and troughs nearest the Atlantic, south 

 eastern 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 k) are more open, 

 each having its western side steepest ; the next (J) 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 

 without confusion. It is also clear that large quantities of rock have 

 been removed by aqueous action or denudation, as will appear if we 

 attempt to complete all the curves in the manner indicated by the dotted 

 lines at i and k. 



The movements which imparted so uniform an order of arrangement 

 to this vast system of rocks must have been, if not contemporaneous, at 

 least parts of one and the same series, depending on some common cause. 

 Their geological date is well defined, at least within certain limits, for 

 they must have taken place after the deposition of the carboniferous 

 strata (No. 5), and before the formation of the red sandstone (No. 4). 

 The greatest disturbing and denuding forces have evidently been ex- 

 erted on the southeastern side of the chain ; and it is here that igneous 

 or plutonic rocks are observed to have invaded the strata, forming dykes, 

 some of which run for miles in lines parallel to the main direction of the 

 Appalachians, or N. N. E. and S. S. W. 



The thickness of the carboniferous rocks in the region c is very great, 

 and diminishes rapidly as we proceed to the westward. The surveys of 

 Pennsylvania and Virginia show that the southeast was the quarter 

 whence the coarser materials of these strata were derived, so that the an- 

 cient land lay in that direction. The conglomerate which forms the gen- 

 eral base of the coal-measures is 1500 feet thick in the Sharp Mountain, 

 where I saw it (at c) near Pottsville ; whereas it has only a thickness of 

 500 feet about thirty miles to the northwest, and dwindles gradually 

 away when followed still farther in the same direction, until its thickness 

 is reduced to 30 feet.* The limestones, on the other hand, of the coal- 

 measures, augment as we trace them westward. Similar observations 

 have been made in regard to the Silurian and Devonian formations in 

 New York ; the sandstones and all the mechanically-formed rocks thin- 

 ning out as they go westward, and the limestones thickening, as it were, 

 at their expense. It is, therefore, clear that the ancient land was to the 

 east, where the Atlantic now is ; the deep sea, with its banks of coral 

 and shells to the west, 0" where the hydrographical basin of the Missis- 

 sippi is now situated. 



In that region, near Pottsville, where the thickness of the coal-meas- 

 ures is greatest, there are thirteen seams of anthracitic coal, several of 

 them more than 2 yards thick. Some of the lowest of these alternate 



* H. D. Rogers, Trans. Assoc. Amer. GeoL 1840-12, p. 440. 



