Appalachian Geosyncline. 243 



greater part of the piedmont remained unaffected save by 

 crustal uplift, since it originated as a consequence of crustal 

 downwarping in front of the mountains and all but its upper 

 deposits were below any possible river base-level. The con- 

 glomeratic members of the Pocono and Pottsville are, how- 

 ever, hundreds of feet thick in eastern Pennsylvania, growing 

 finer and thiuner westward, but covering originally many 

 thousands of square miles. Their volume implies, therefore, 

 the former presence of a piedmont as wide and as high above 

 sea level as has been indicated by the other lines of evidence. 

 The theory of the relations of climatic cycles to cycles in 

 sedimentation converges to the same conclusion as the other 

 lines of evidence regarding the former existence of a wide pied- 

 mont slope constituting an eastern side to the Appalachian 

 geosyncline. 



Volume and Sources op Upper Devonian Sediment. 



Of the volume of sediments deposited in the Appalachian 

 geosyncline during Middle and Upper Devonian time, Willis 

 states : 



"If this mass with approximately the dimensions with which 

 it was deposited in the sea, could be restored upon a sea-level 

 plain of Appalachia, it would constitute a mountain range closely 

 resembling in height, extent and mass the Sierra Nevada of 

 California."* 



The writer has computed from the data shown in fig. 1 

 (p. 89) the original volumes of sediment within an area bounded 

 by lat. 39° 30' on the south, 43° on the north, and from the 

 original eastern margin to long. 80° 30' on the west. Within 

 this area the map by Willis indicates a volume of 55,000 cubic 

 miles. The Middle Devonian comprises about one-fifth of 

 this, leaving 44,000 cubic miles for the Upper Devonian. On 

 the map accompanying the present article there is shown 

 within these boundaries 63,000 cubic miles of Upper Devonian 

 sediment, an increase of 43 per cent over the estimate by 

 Willis. Allowing for the extension which the present writer 

 has given to the north the present estimate becomes readily as 

 much as 50 per cent greater than the previous. This is an 

 impressive measure of the volume of the adjacent land which 

 was eroded in Upper Devonian times. But it is a minimum 

 measure, since that part of the rocks which was taken into 

 solution was carried farther away, and of the mechanical sedi- 

 ments it represents only that part which was carried westward 

 into the trap of the geosyncline. 



* Paleozoic Appalachia, Md. Geol. Snrv.. vol. iv, Pt. I, p. 62, 1900. 



