312 



H. WILLIAMS — STRUCTURE OF THE PIEDMONT PLATEAU. 



the same easterly dip the thick beds of sand- 

 stone which compose Sugadoaf mountain. 

 These thin out toward the north to a few insig- 

 nificant sandstone patches, while toward the 

 south they soon disappear beneath the Newark 

 transgression. The Sugarloaf sandstone passes 

 on its eastern side upward by a gradual tran- 

 sition through shaly layers into sandy slates, 

 and these again into the succession of sericite 

 and chlorite schists, which compose the mass 

 of the semi-crystalline area. Beneath the sand- 

 stone the shales are more disturbed, and, as there 

 is here no such transition, this surface may rep- 

 resent a fault or thrust, as suggested by Mr. 

 Keyes.* 



The main body of semi-crystalline rocks in 

 the Piedmont region are slatesand soft schists 

 and, north of the Baltimore and Ohio main 

 stem, narrow limestone bauds, which preserve a 

 constant north to north-northeast strike. The 

 dip of these rocks is always toward the east as 

 far as a line (the axis of the fan) which runs 

 nearly north from Great Falls, where they 

 become vertical. Toward this axis the dip be- 

 comes constantly steeper and steeper, yet the 

 character of the rocks is but little changed save 

 that they become gradually more shattered and 

 crinkled. 



After passiug the vertical axis, the same 

 structure is observed on the east as on the west, 

 but in an inverse order. The dip of the strata 

 turns gradually toward the west, becoming less 

 and less steep as we proceed from the axis 

 toward the coastal plain. As may be seen 

 from the map, all the westerly dipping strata 

 in the southern part of the Piedmont region, 

 where the axis is nearly coincident with the 

 boundary between the eastern and western 



*The course of the great Triassic trap dike, which extends 

 from Emmitsburg entirely across the state (see map), is worth 

 noticing in this connection. It is nearly parallel to the >yn- 

 clinal axis of the Piedmont plateau, and may represent a 

 preexisting line of weakness in the crust. 



M 



