94 J. Barrell — Upper Devonian Delta of the 



In the past two decades has grown up an increasing apprecia- 

 tion of the complexity of erosion cycles. Davis early recog- 

 nized one baselevel as the floor of the Triassic formation and 

 another as the surface truncating the higher ridges of Penn- 

 sylvania. The latter he correlated with the floor at the base 

 of the Potomac group and therefore it was placed by him as of 

 Cretaceous date. Another erosion level, existing in the val- 

 leys and beveling the softer formations, he placed as of Ter- 

 tiary age. Since then still other minor cycles have been added. 



What effect should this development of the erosion theory 

 have upon the problem of Paleozoic geography ? Its influence 

 may be considered by introducing first the effect of the Cre- 

 taceous baseleveling. In that Jurassic-Cretaceous cycle it has 

 been held that all of eastern North America except certain 

 residual mountain groups was reduced to a peneplain. Con- 

 sequently if ancient strata, for example those of the Devonian 

 age, were warped upward at the close of the Paleozoic above 

 the level of the Cretaceous peneplain, no matter how far they 

 originally extended, so long as they did not enter into the 

 residuals still remaining upon that plain, they would have 

 become completely destroyed. Even if they had extended 

 outwards for hundreds or thousands of miles at elevations 

 above that level, now no vestige would remain. Consequently 

 the extension of the strata to the present level of the uplands 

 would only restore the boundaries as they existed in the Cre- 

 taceous, not in their time of origin. This may be called an in- 

 terpretation of paleogeography with respect to a three-cycle 

 basis, the Triassic and Tertiary cycles being looked upon by 

 most writers as relatively partial and local. 



Chamberlin and Salisbury in their Geology and Willis in his 

 Outlines of Geologic History have recognized the pertinency 

 of this principle by showing a probable extension of the early 

 Paleozoic seas upon the Canadian shield where now only pre- 

 Cambrian rocks exist below the Cretaceous level. Such rela- 

 tions of intervening erosion cycles to the problem of the ancient 

 boundaries does not however appear to have been clearly 

 formulated. A paper in which is stated distinctly the relation 

 of the Cretaceous cycle to an earlier problem, here the problem 

 of Permian folding, is that by K. T. Chamberlin on the 

 Appalachian Folds of Pennsylvania.* Here the Cretaceous 

 baselevel is definitely assumed to be essentially the same as the 

 post- Permian baselevel. t 



It is the conclusion of the present writer, however, that 

 many erosion cycles, developed with respect to successive base- 

 levels, have intervened between the Devonian and the present 

 and that the Cretaceous cycle is a broad term for a number of 



* Jour. Geol., xviii, 228-251, 1910. fLoc. cit., 237-241. 



