336 E. B. MATHEWS — MARYLAND AND PENNSYLVANIA PIEDMONT 



and closest similarity with the Wissahickon schists of the type locality 

 near Philadelphia. The formation in this part of its development 

 broadens from a narrow band at the Susquehanna river by increased 

 folding about the anticlines of Baltimore gneiss and synclines of Cockeys- 

 ville marble into a belt 10 to 15 miles broad as it crosses the Northern 

 Central railroad. From this point it narrows somewhat to the south- 

 westward, and the area is occupied in large part by the large mass of 

 granite passing from Sykesville south westward to Washington and ex- 

 tending thence many miles southward into Virginia. 



North of the phyllite syncline occurs a corresponding mass of more 

 crystalline Wissahickon schist. When, however, this is compared with 

 the rocks on the southern limb of the synclinorium it is found that these 

 rocks average slightly less crystalline and less metamorphosed than the 

 corresponding rocks on the south. That they represent the same horizon 

 seems to be well established by the areal distribution of the various 

 masses, although it has been found impossible to carry the mapping of 

 individual beds more than a few miles along the strike, and hence it has 

 seemed inadvisable to attempt detailed cartographic representation. The 

 Wissahickon schists on the west side of the syncline of ph3 r llites passes 

 southwesterly across the state, narrowing considerably in the southern 

 portion of Carroll county and widening somewhat in passing southward 

 to the Potomac river. The areal mapping of this region has been carried 

 out in considerable detail by Mr Keith, of the United States Geological 

 Surve}', who has shown that the line of separation between the eastern 

 and western portions of the Piedmont plateau, as laid down by Williams,* 

 is in reality a sharply crenulate line, due to the folded structure of the 

 area, which brings the contact between the phyllites and the more crys- 

 talline schists repeatedly to the surface. 



The band of phyllite, forming a synclinal trough extending from the 

 Susquehanna southward, enters the state from York county, Pennsyl- 

 vania, continues as a belt, varying from 5 miles in thickness at the Susque- 

 hanna to about a mile at Whitehall, on the Northern Central railroad, 

 whence it gradually widens southward to an average breadth of 3 miles 

 in the southern part of Carroll county. The areal distribution indicates 

 that we have a synclinal trough of considerable extent and well defined 

 character, which is buckled at its center, and plunges northeastward and 

 south westward, reaching its maximum depth in the vicinity of Delta, 

 Pennsylvania, where the Cardiff quartzite-conglomerate and Peachbot- 

 tom slates are found folded within it. The southern termination of this 

 phyllite belt has not been mapped in detail, and the limits here given 



* Bull. Geol. Soc. Am., vol. 2, 1892. 



