Mathews — Structure of the Piedmont Plateau. 141 



Art. XI. — The Structure of the Piedmont Plateau as 

 shown in Maryland; by Edward Bennett Mathews. 

 (With Plate X.)' 



The structure of the more or less metamorphosed sedimen- 

 tary and igneous rocks exposed in the Piedmont Plateau, lying 

 between the Blue Pidge on the west and the Coastal Plain on 

 the east along the Middle Atlantic coast from New York south- 

 ward, is a geological problem of wide interest to American 

 geologists and has occasioned more or less discussion among 

 various investigators who have worked upon it. 



Throughout the Piedmont region are exposed numerous 

 highly-crystalline gneisses and schists intermingled with crys- 

 talline limestones, quartzites, and phyllites which have been 

 intruded by large and small igneous masses of granite, gabbro, 

 serpentine, and volcanic rocks ; all of which have been meta- 

 morphosed in varying degree up to the point where they have 

 lost all evidence of their original condition. 



The deciphering of the various formations occurring within 

 the Piedmont is still in progress and many areas are yet 

 unstudied, but the areal distribution of the various formations 

 throughout the region north of Virginia has been determined 

 with sufficient accuracy to indicate the various types of rock 

 present. The areas about Washington, Philadelphia, and a 

 large portion of the intervening country within the limits of 

 the State of Maryland, have been studied in detail until it 

 seems that some clue has been gained regarding the broad 

 structural features and geological relationship of this compli- 

 cated region. In the present paper it is proposed to give a 

 tentative interpretation of the broader structural features as 

 shown in Maryland by the detailed mapping of the author and 

 his assistants during the last seven years. The basis of work* 

 includes a reconnaissance of the entire Maryland Piedmont 

 and the detailed mapping of approximately 1250 square miles 

 lying north and east of Baltimore. 



General Characteristics of the Piedmont. 



The crystalline rocks of the Piedmont Plateau are well 

 exposed in the area about New York, w T here they have been 



* While the work in Maryland has been in progress, the author has had 

 many office and field conferences with Dr. Bascom, who has generously 

 allowed him to examine and to use her detailed manuscript maps of the 

 Philadelphia area. The work in Maryland has been conducted independ- 

 ently at the same time and both investigators have been led to similar results 

 for their special regions. The application of their common interpretation to 

 the whole area of the Eastern Piedmont is here first issued by the author, 

 who wishes to acknowledge the assistance received from his knowledge of 

 Dr. Bascom's results which have most generously been placed at his dis- 

 posal. 



