WUJJAM8.] THE APPALACHIAN REGION. 271 



The relation between the topography and drainage of the country 

 and the geological structure is nowhere more apparent than in the 

 Appalachian region, as was long ago pointed out by Lesley. 11 The 

 drainage is consequent to the post-Carboniferous folding. The rivers 

 in all probability flowed in the synclinal valleys. Many of the smaller 

 streams follow the direction of the' folds, while the larger ones, like the 

 Potomac Schuylkill, and Susquehanna, owe their transverse course to 

 mutual reaction and adjustment through repeated cycles of elevation, 

 tilting, and depression. This subject has been recently treated by 

 Prof. William M. Davis in his studies of the rivers and valleys of 

 Pennsylvania." 



The Piedmont Plateau. — For a distance of 43 miles after leaving Wash- 

 ington the railroad traverses, in a northwest direction, a rather low and 

 rolling country, before entering the mountain belt proper at the station 

 called Point of Pocks. it is composed of gneisses and mica-schists. 

 Sericite and chloritic schists, marbles and quart/ites. whose strike fol- 

 lows in tin- main the general Appalachian trend. The relief of this 

 country is given it by rapid streams or torrents, which are still exca- 

 vating deep, rocky channels. The section of this belt affords a good 

 idea of its general character, for although it increases greatly in width 

 farther south, it everywhere retains a constant character at the east- 

 ern base of t he Appalachians, and is for this reason appropriately desig- 

 nated the Piedmont plateau. 



Topographically the Piedmont Plateau in Maryland and Virginia 

 begins at the Catoctin .Mountain, which meets the Potomac at Point of 

 Rocks and pursues a Straight course, across the former State, nearly 

 northward from that point: geologically, however, the peculiar forma- 

 tions of still undetermined age, which are most characteristic of this 

 region, begin farther to the east. At the base of Catoctin Mountain 

 stretches a broad transgression of Triassic (Newark) red sandstone. 

 which iscrossed by the railroad at its narrowest point. From beneath 

 the eastern border of this emerge the upturned edges of the Frederick 

 Valley limestone, which has recently been found from its fossils to be 

 the same as the Trenton-Chazy limestone which forms the valley 



farther west. Past of this, with constant easterly dip. Bucceed over- 

 lying slates and another ridge of sandstone, which, however, only 

 assumes topographical importance, as a high ridge, in the isolated 

 mass of Sugarloaf. This mountain, seen just north of the railroad, is 

 a thick monocline of easterly dipping beds, 1,360 feet in height. Geo- 

 logically this mass forms the western boundary of the Piedmont 

 plateau in Maryland. To it succeeds that vast Complex of semicrystal- 

 line and holocrystalline rocks whose origin, age, and structure repeated 

 earth movements have rendered most obscure. Along the section 

 these extend eastward, becoming more and more crystalline, until they 

 are buried beneath the overlying deposits of the Coastal plain. 



