330 E. B. MATHEWS — MARYLAND AND PENNSYLVANIA PIEDMONT 



During the course of the work in the Maryland area, extending over a 

 period of ten or twelve years, there have been frequent conferences with 

 Miss Bascom, who has been engaged in investigations in the Piedmont 

 area, and the knowledge of the results there gained have been gener- 

 ously placed at the disposal of the writer at all times. The conclusions 

 reached, independently in the Pennsylvania and Maryland areas, are 

 found to be in practical accord, and it is the purpose of the present paper 

 to show the harmony of interpretation in these two areas and to discuss 

 the southward continuation of the various formations and structures 

 described by Doctor Bascom in the preceding paper. 



Formations of the Piedmont 

 list of the forma tions 



In a paper previously published by the author* it was shown that 

 there are within the Maryland area six well defined sedimentary forma- 

 tions and numerous rocks intruded in them. The sedimentary forma - 

 tions are as follows : 



f Peachbottom slates. 



I Cardiff quartz conglomerate 



Ordovician ? <{ Wissahickon mica-schist, including 



phyllite with intercolated lime- 



(, stone bands at the top. 

 Cambro-Ordovician (Shenandoah) ? . Cockeysville f marble. 



Cambrian Setters f quartzite. 



Pre-Cambrian Baltimore gneiss. 



BALTIMORE GNEISS 



The oldest formation in Maryland is the Baltimore gneiss, which oc- 

 curs in several well defined areas in the eastern portion of the Piedmont 

 between the Susquehanna and Potomac rivers. The easternmost of these 

 Baltimore gneiss occurrences is within the area of Cecil county, east of 

 the Susquehanna river, studied by Doctor Bascom, and extends from this 

 point southwestward, widening to an area of 5 miles or more in breadth 

 where it is overlain 03* Coastal Plain deposits. This formation is limited 

 on either side by igneous rocks. An outlier a mile or less in width, 

 extending for several miles southwestward from the Susquehanna river, 



* The structure of the Piedmont plateau as shown in Maryland. Araer. Jour. Sci.. 4th ser., vol. 

 xvii, 1904, pp. 141-159. 



fThe terms Cockeysville and Setters are old terms employed by Williams, which are here used 

 pending the final decision on names by the Committee on Formation Names of theU. S. Geological 

 Survey. It is not thought that they represent any different horizons from the Chickies quartzite 

 and Chester Valley limestone of the Philadelphia area. 



