238 ROLLIN T. CHAMBERLIN 



attitude of the land surface before the folding took place; the second 

 requirement is a visible base-line available at the present day, above 

 which the projection of the folded tract is to be measured. In view 

 of the fact that the critical period of folding occurred as far back in 

 geologic history as the close of the Paleozoic, and in view of the still 

 more compromising fact that other dias trophic movements have 

 disturbed the Appalachian belt since that time, it might at first seem 

 that such planes of reference could not reasonably be hoped for. 

 But fortunately the physiographic history of the Appalachians has 

 been admirably arranged to meet the requirements of the case. 



The last beds to be deposited in the present mountain tract were 

 the upper strata of the Coal Measures. From the nature of these 

 beds, particularly the persistence of coal repeated in a considerable 

 number of separate seams over adjacent areas of wide extent, it is 

 confidently inferred that the Appalachian region during the Upper 

 Carboniferous must have constituted an almost perfect plain of 

 sedimentation, at times just at, or a few feet above, the sea-level, and 

 at other times but slightly submerged. The very considerable areas 

 over which individual coal-seams may be traced testify to the uni- 

 formly level condition of the region. It was this rather remarkable 

 plain of sedimentation, warped and wrinkled by the throes of the 

 dying Paleozoic, that rose into the great Appalachian plications. 



On the other hand, the mountains thus formed had their day and 

 were gone before the end of the Mesozoic, when a new plain had been 

 established. The Kittatinny base-level, strikingly visible even today 

 in the level crest-lines of all the major ridges of this portion of Penn- 

 sylvania, shows that by the close of the Cretaceous, the former 

 mountain tract had been beveled till the land again stood close to the 

 sea-level. 



Thus the Appalachian region started approximately from the 

 sea-level, was thrust up into tremendous folds, and then planed down 

 by erosion again essentially to the level of the sea. If no other 

 diastrophic movements intervened to complicate the case, the total 

 upwarping of the crust should be expressed by the average height 

 of the freshly folded tract above the sea, and if the height of the sea 

 relative to the land of this area remained the same till .the Kittatinny 

 base-plain was established, that plain should constitute an absolute 



