RESUME 



327 



cealecl rock floor to alter them when subsequently the rock floor was 

 discovered. 



The Wissahickon and the Schuylkill, crossing the easily eroded lime- 

 stone belt and cutting gorges in hard gneisses, are examples of super- 

 imposed streams. The peculiar courses of Gulf and Valley creeks, on 

 the other hand, are due rather to stream robbery. 



Each of these streams originally pursued direct northeast courses to 

 the Schuylkill. Their headwaters were captured by tributry streams to 

 the Schuylkill possessing higher gradients. Valley and Gulf creeks 

 therefore turn nearly at right angles to their former courses and pursue 

 northwest courses. The beheaded remnants of these streams are Trout 

 and Mechanicsville creeks. 



The secondary tributaries do not show the characters of superimposed 

 streams, but are adjusted to the constitution of their rock bed. This 

 fact and differential weathering explain the minor diversities of topog- 

 raphy which have been noted in that portion of the plateau in which 

 the Paleozoic and pre-Paleozoic series are uncovered. 



Resume 



The crystalline formations of the Piedmont district of Pennsylvania 

 are a pre-Cambrian gneiss, a Cambrian quartz-schist, a Cambro-Ordo- 

 vician marble, and an Ordovician mica-gneiss and mica-schist, together 

 with intrusive granitic and gabbroitic igneous rocks. 



The dynamic forces which have developed these formations from 

 simple sediments have folded them in anticlinoria and synclinoria, with 

 major axes striking northeast-southwest and minor axes transverse to 

 the major axes. 



Following this folding, which was accompanied by thrust faulting, 

 occurred pre-Triassic erosion, succeeded by Triassic estuarine deposition. 



The period of elevation which followed was marked by the production 

 of the Jurassic peneplain, on which was subsequently spread the de- 

 posits of Cretaceous, Tertiary, and Quaternary time, separated by brief 

 erosion intervals. 



With the final elevation of the Jurassic peneplain began its dissection 

 and the development of the present topography. 



Addenda 



Since this paper has been put in type a field conference has been con- 

 ducted with Messrs A. Keith, G. 0. Smith, and E. B. Mathews with a 

 view to determining the age of the Wissahickon mica-gneiss. The results 

 of this conference, while establishing the Ordovician age of the Wissa- 



