AN ANCIENT AND PROFOUND UNCONFORMITY. 317 



however, as little in accord with the fiicts as the first, since it cannot possibly 

 be reconciled either with the conformity in dip and strike of the schists and 

 gneisses along their contact, or with the infolding of the slates and schists in 

 the gneisses, as may be seen in the Peach Bottom-Delta area and at Occo- 

 quan, Virginia. 



We are therefore driven to the third hypothesis as the most reasonable 

 explanation of the facts. This supposes an ancient and crystalline floor, 

 upon which were deposited the sediments now formiug the western slates, 

 sandstones, limestones and schists. At the time of the Appalachian uplift 

 this crystalline floor underwent a final folding, which involved the overly- 

 ing sediments, and thereby folded, faulted, cleaved and altered them. This 

 hypothesis seems to account for the difference between the rocks of the two 

 areas and for the abruptness of their contact, while at the same time it ex- 

 plains the conformity along this contact, and the fact that this boundary 

 and the axis of the synclinal or fan are not coincident. We may therefore 

 accept it as the most probable one unless future exploration shall render 

 some modification of it necessary. 



As to the age of the eastern and western Piedmont areas in Maryland, we 

 can as yet offer no. definite conclusions. It seems most probable that the 

 boundary of the Paleozoic should be pushed eastward, not merely past the 

 Frederick limestone but quite to the limit of the semi-crystalline schists, in 

 which case the holocrystalline rocks below them would be assigned to the 

 Algonkian or Archean. But the proof of this and the working out of many 

 interesting details remain for future exploration, which it is hoped will decide 

 on paleontological evidence many points now^ in doubt. 



DISCUSSION. 



Professor W. M. Davis : I would ask if the line of contact between the 

 Archean to the east and the Triassic (Newark) beds to the west is a line of 

 faulting? It will be noted that there is an apparent resemblance in the 

 Piedmont section described by Dr. Williams to the fan structure of the Alps. 

 Both the fan structure and faulting suggested by the section indicate enor- 

 mous denudation of overlying masses. The region seems to represent the 

 base-level of erosion of an old mountain range that has been elevated per- 

 haps the twentieth time, and is now again being eroded. 



Major Jed Hotchkiss : I have in my possession an unpublished section 

 by Professor W. B. Rogers, which strikingly resembles that constructed by 

 Professor AVilliams. I wish to call particular attention, however, to certain 

 topographical features of the Piedmont plateau region. There is on the west, 

 the Appalachians ; to the east of this, the coastward range, which, however, 



XLVII— Bum.. Gkol Soc. Am., Vol. 2, 1890. 



