Ch. XXV.] APPALACHIAN CHAIN. 499 



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 car- 

 boniferous strata (No. 5), and before the formation of the red sand- 

 stone (No. 4). The greatest disturbing and denuding forces have evi- 

 dently been exerted 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, not expressed in the section, some of which run 

 for miles in lines parallel to the main direction of the Appalachians, or 

 N.NE. 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 ancient land lay in that direction. The conglomerate which 

 forms the general 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 direc- 

 tion, until its thickness is reduced to 30 feet.* The limestones, on 

 the other hand, of the coal-measures augment as we trace them west- 

 ward. Similar observations have been made in regard to the Silurian 

 and Devonian formations in New York ; the sandstones and all the 

 mechanically-formed rocks thinning 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, or where 

 the hydrographical basin of the Mississippi is now situated. 



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

 measures is greatest, there are thirteen seams of anthracitic coal, sev- 

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

 alternate with beds of white grit and conglomerate of coarser grain 

 than I ever saw elsewhere, associated with pure coal. The pebbles of 

 quartz are often of the size of a hen's egg. On following these pudding- 

 stones and grits for several miles from Pottsville, by Tamaqua, to the 

 Lehigh Summit Mine, in company with Mr. H. D. Rogersj in 1841, he 

 pointed out to me that the coarse-grained strata and their accompany- 

 ing shales gradually thin out, until seven seams of coal, at first widely 

 separated, are brought nearer and nearer together, until they succes- 



:: II. D. Rogers. Trans. Assoc. Amer. Geol., 1840-42, p. 440. 



