314 G. H. WILLIAMS — STRUCTURE OF THE PIEDMONT PLATEAU. 



(3) the extensive superficial decay to which they have been subjected. As 

 we approach their eastern boundary the semi-crystalline rocks exhibit the 

 effects of more intense dynamic action. Along the axis, where they stand 

 vertical, and also east of it, they are much broken, crinkled, and corrugated. 

 Still, all the disturbance and alteration observed in the semi-crystalline 

 schists may be readily accounted for by a single earth-movement ; i e., by a 

 force acting for a long time in a single direction. 



The rocks of the eastern area, as the preceding petrographical descriptions 

 have shown, are, in spite of a certain correspondence of sedimentary types, 

 broadly distinguished from the schists and slates of the western area. They 

 have, indeed, by the most complete metamorphism and recrystallization, lost 

 nearly all traces of any clastic structure which they may once have possessed. 

 This general distinction is admirably illustrated by Mr. Keyes in his two 

 figures of the microscopic appearance of the quartz-schist of the eastern 

 area, and a sandstone of the western area (this volume, page 321). These 

 rocks may once have been nearly identical, but, if so, the former has lost 

 its original structure quite completely. 



The much greater variety of rocks within the eastern area is largely due 

 to the ancient eruptives, which are there so abundant. But these hard and 

 resistant masses have suffered hardly less complete foliation and metamor- 

 phism than the sediments which surround them, while in both this action is 

 far in excess of what has taken place in any portion of the western area. 



Another point of great importance is the abruptness of the passage from 

 the semi-crystalline to the holocrystalline rocks. The schists at AVestrain- 

 ster are hardly more crystalline than those bordering the Frederick valley, 

 while so soon as we pass the boundary line between the two areas we meet 

 gneisses as granitoid and perfectly crystalline as any to be found within the 

 whole eastern district. 



The holocrystalline rocks occupy all the Piedmont area in Maryland east 

 of the eastern base of Parr's ridge, except the infolded and overturned mass 

 of soft schists surrounding the Peach Bottom and Delta roofing-slates, which 

 descend in a south-southwesterly direction from York and Lancaster coun- 

 ties, Pennsylvania* (see map, plate 12). Throughout all of the Piedmont 

 area in Maryland east of the axis there is the general tendency to westerly 

 dip above alluded to, and yet this feature is so much less constant in the 

 holocrystalline than in the semi-crystalline rocks that it indicates a structure 

 added to others which it has only partially obliterated. The very irregular 

 areas occupied by the different rocks, the abrupt changes in trend and struct- 

 ure, and the much more intense alteration of the sedimentary beds, all bear 



* There is a similar oeeurrenee of slates between Oecoqnan and Qiiantieo, in Virginia, apparently 

 wholly on the eastern side of the crystallines; but liere the rocks lying still farther eastward are 

 buried beneath the formations of the coastal plain. At Occoquan the granite west of the slates is 

 intrusive into them. 



