Mathews — Structure of the Piedmont Plateau. 159 



ite of Cambrian age ; an intermittent dolomitic marble or mag- 

 nesian limestone of Cambro-Ordovician age ; and a series of 

 mica-shists and the gneisses of Ordovician age. Above these 

 occur a somewhat intermittent poorly developed quartzitic con- 

 glomerate and the Peach Bottom slates. 



3. The igneous rocks consist of an immense gabbro sheet, 

 intruded by numerous large bodies of granite and meta-rhyolite, 

 and accompanied by numerous more basic serpentinized bodies. 

 These various masses represent stages in a single extended 

 period of igneous activity. 



4. The time when this activity took place was later than 

 early Silurian and earlier than the late Carboniferous; probably 

 in the early part of this interval. 



5. The chief structural features of the region are the meta- 

 morphism and constant schistosity and the broader folding of 

 the different rocks. 



6. The metamorphism of the rocks, especially of the banded 

 gneisses, probably commenced prior to the intrusion of the 

 gabbro and granite and was accentuated by them in the eastern 

 portion of the Plateau. 



7. The folding of the region is of the Appalachian type, the 

 rocks occurring in several long, more or less parallel folds, with 

 few faults and but occasional overturned folds. 



8. The eastern and western areas are probably of the same 

 age ; differences in metamorphism being due to the large bodies 

 of deep-seated intrusives on the east and the smaller bodies of 

 surface volcanics on the west. 



9. The sequence found in Maryland may be recognized from 

 Washington to Trenton and in the region north of New York. 



If these conclusions are confirmed by later investigation, it 

 will be necessary to modify the generally accepted hypothesis 

 of a former mountain range along the eastern Atlantic now 

 represented by the Piedmont Plateau ; or the location of this 

 hypothetical range, which is supposed to have supplied the 

 sediments for the Appalachian sea during Paleozoic time, must 

 be shifted, at least for the earlier Paleozoic, farther east where 

 its roots would not lie buried under Coastal Plain deposits. 



Johns Hopkins University, November, 1903. 



