Appalachian Geosyncline. 93 



glomerate marine. The beds in Pennsylvania are shown as 

 wholly marine and advancing east somewhat farther than 

 indicated on previous maps. 



The present map, fig. 1, shows the shore line at the close of 

 the Devonian farther west than any previous map but the 

 margin of the sediments farther east, except for the New 

 Jersey strait of Schuchert which is here eliminated. Between 

 the two limits is shown, not a shallow sea or estuary but a 

 subaerial delta plain. The comparison with the previous maps 

 crystallizes the differences between them and introduces the 

 need for a discussion of the principles which have controlled 

 the past and the present restoration. 



Influence of Erosion Cycles. 

 Present TAmits Dependent upon Intervening Baselevels. 



The original development of a formation was dependent 

 upon the baselevel of the time, the initial baselevel in its his- 

 tory. At a distance from the shore the inland baselevel deter- 

 mining the relations of erosion and continental sedimentation 

 are controlled by the river grades and may depart appreciably 

 from sea level. The limits of deposition, however, are limited 

 in the initial cycle to the parts below baselevel even if some- 

 what above sea level. 



In the drawing of the earlier paleogeographic maps the views 

 prevalent in the middle of the last century determined the 

 principles of construction. It was originally assumed that the 

 present limits of a formation are controlled by but two base- 

 levels of erosion, the initial one, giving the original limits of 

 sedimentation, thought of as necessarily a shore line ; and the 

 present cycle, giving the existing relief to the land. The idea 

 of clearly separated baselevels was not however present in 

 theory. Uplift was looked upon as progressive, the successive 

 shore lines forming on the whole a descending series. It 

 apparently was not thought that any part of the continental 

 surface save the anticlines of folded regions had lost very much 

 in elevation. No quantitative comparison had been made 

 between the great volumes of the sediments and the enormous 

 denudation which they imply. Consequently the old shore 

 lines were located by filling up in imagination the river valleys 

 and extending the strata to the higher upland levels of the 

 region. It was not believed that extensive overlaps or outliers 

 could have existed, for it was thought such could not have been 

 wholly eroded. This blended two-cycle principle, inherited 

 from the past, still unconsciously controls the work of many 

 geologists whose specialty does not lie in the application of 

 present physiographic knowledge to the distant past. 



