290 F. RASCOM — PIEDMONT DISTRICT OF PENNSYLVANIA 



Page 



Igneous rocks 308 



Classification 308 



Granite-gneiss 308 



Distribution 308 



Character '. 309 



Gabbro (gabbro, hypersthene-gabbro, norite, meta-gabbro) 311 



Distribution 311 



Character 312 



Serpentines (meta-pyroxenite, meta-peridotite, and other alteration 



products) 316 



Distribution 316 



Character 316 



Meta-gabbro 318 



Distribution 318 



Character 319 



Hornblende-gneiss 319 



Diabase „ 320 



Pegmatites 1 322 



Geologic history of the Piedmont district of Pennsylvania 322 



R6sume 327 



Addenda 327 



Geography of the Piedmont 



general relations 



Of the three physiographic provinces into which the Atlantic border 

 region is divisible — the Appalachian district, the Piedmont plateau, and 

 the Coastal plain — the Piedmont plateau preserves the record of the 

 longest and most varied geologic and physiographic history. 



The plateau lies at the southeastern foot of the Appalachian system 

 and is separated from the Atlantic ocean by a belt of coastal plain of 

 variable width and from the edge of the continental plateau by a belt of 

 coastal province possessing a uniform width of 200 miles. 



The Piedmont district extends north and south from Maine to Ala- 

 bama, with an average width of 50 miles. Its western limit is defined 

 by the eastern slopes of the Blue ridge; its eastern boundary is defined 

 by an equally conspicuous change in topography, the abrupt transition 

 from a diversified upland to an undiversified lowland. 



Eastward from this boundary the navigable streams, opening into tidal 

 estuaries, afford good shipping facilities. Westward the streams cease 

 to be navigable and occupy rocky channels. At the head of navigation 

 and on the boundary between plateau and plain are situated many of 

 the large cities of the Atlantic states. 



In general the plateau is a level upland of moderate elevation, sloping 



