142 Mathews — Structure of the Piedmont Plateau. 



studied by Dr. F. J. H. Merrill* and his colleagues. South- 

 ward from the JSTew York area the older crystalline rocks of 

 the Piedmont are covered by the formations of the Jura-trias 

 and do not appear again until in the vicinity of Trenton, whence 

 they pa&s southwestward across Pennsylvania into Maryland, 

 offering numerous good exposures along the banks of the Dela- 

 ware, Schuylkill, and Brandywine, where they have been 

 investigated by members of the Pennsylvania Survey and 

 more recently in great detail by Dr. Bascom. Within the 

 limits of Maryland the Piedmont crystallines are trenched by 

 the Susquehanna, Gunpowder, Patapsco, Patuxent, and Po- 

 tomac rivers and their tributaries. Beyond the Potomac, the 

 same formations are continued southward through Virginia, 

 North and South Carolina, Georgia and Alabama, where they 

 have been examined by members of the different State surveys 

 and by Mr. Arthur Keith of the U. S. Geological Survey. 



In each of the areas studied, about New York, Philadelphia, 

 Baltimore, and Washington, similar series of rocks have been 

 found but the interpretation of their relationship has varied 

 somewhat, and until recently no attempt has been made to 

 determine the structure of the territory. The following table 

 represents the lithologic types recognized in the different 

 regions by the various authors. 



From the following table it is easily seen that four distinct 

 types of rocks have been recognized in each of the areas, though 

 not always represented on the maps accompanying the verbal 

 description. These are (1) a banded gneiss, (2) a thin-bedded 

 or arkosic quartzite, (3) a marble, and (4) a series of mica 

 schists. 



Banded Gneiss. — In each of the areas is a highly crystalline 

 gneiss composed of quartz, feldspar and mica with accessory 

 minerals so distributed as to produce a well-marked gray banded 

 gneiss, the individual bands of which vary from a fraction of 

 an inch upward, the average thickness, however, being quite 

 small. Some of these beds are highly quartzose resembling a 

 micaceous quartzite, others are rich in biotite or hornblende 

 producing dark to black bands indistinguishable in a hand speci- 

 men from mica and hornblende schists and gneisses derived 

 from igneous rocks by metamorphism. Through these banded 

 gneisses are intruded pegmatite and aplitic dikes more or less 

 parallel to the regular banding of the gneiss. In all of the 

 areas these beds are found highly inclined in their banding, 

 which represents original variation in composition of the sedi- 

 ments or the igneous rocks from which they were formed. 

 Some authors have regarded these as sedimentary, some as 



* N. Y. Folio, Geologic Atlas of the United States, Folio No. 83, Wash- 

 ington, 1902. 



