108 •/. Barrel! — Upper Devonian Delta of the 



and the anticlinorium would show a complex of older and crys- 

 talline rocks thrust over the Mesozoic and Tertiary formations 

 on the flanks. 



In the Rocky Mountain system examples of sedimentary 

 mantles may be seen in the Bighorn Mountains, the Snowy 

 Mountains north of the Yellowstone Park, and in other ranges. 

 The formations which can be asserted never to have existed 

 on the Rocky Mountain ranges are in general those, like the 

 Laramie and the Tertiary, which have been deposited after the 

 mountains began to grow and show by their nature that they 

 were derived from the mountain waste. Yet if these Cordil- 

 leran movements had taken place in the Paleozoic, erosion 

 would since have widely removed the pre-orogenic sedimentary 

 formations from the regions of uplift and an application of 

 this principle used by Willis would result in the drawing of 

 Paleozoic and Mesozoic shore lines where none existed. 



We may turn next to the Appalachians and interpret them 

 according to these other examples. The folds of the Great 

 Valley dip southeastward, underthrust beneath the older and 

 metamorphosed terranes which constitute the Appalachian 

 mountains and the Piedmont Plateau. The metamorphic 

 province is structurally an anticlinorium which received its 

 overthrust from the back-land of Appalachia. The Great Val- 

 ley, with its parallel ranges and fragments of plateaus, is the 

 synclinorium. But the erosion surfaces of Mesozoic and Ter- 

 tiary origin bevel across both provinces, recognizing only the 

 hardness of the formations and showing only secondary rela- 

 tions to the rock structures. From the analogy of other 

 mountain systems it cannot be held that the formations now 

 restricted to the Great Valley have never existed beyond it. 

 The geosyncline of deposition and the synclinorium of folding 

 are not sjmonymous, nor coincident in outline. Those parts of 

 the higher formations lying on the anticlinorium would have 

 been destroyed before the present time because of upthrust to 

 elevations higher than the now remaining mountains. The 

 coarse and thick sediments of the later Paleozoic show that 

 extensive movements took place in Appalachia. But these 

 movements may have been east of the present Atlantic coast 

 line and the mantles of waste to which these movements gave 

 rise may have extended far east of the present outcrops. How 

 far is to be determined later. 



Let a supposed relation between the synclinorium of the 

 Great Valley and the original limits of the Upper Devonian 

 be tested further by applying the same principle to the still 

 uneroded portions of other Appalachian formations. It will 

 be observed that erosion to sea level, the completion of the 

 present cycle, would remove the Pocono sandstone from the 



