BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 2, pp. 301-322, PL. 12 March i5, 189i 



THE PETROGRAPHY AND STRUCTURE OF THE PIEDMONT 

 PLATEAU IN MARYLAND. 



BY GEORGE HUNTINGTON WILLIAMS OF JOHNS HOPKINS UNIVERSITY. 

 WITH A SUPPLEMENT ON 



A GEOLOGICAL SECTION ACROSS THE PIEDMONT PLATEAU IN 



MARYLAND. 



BY CHARLES R. KEYES. 

 (Read before the Society December 30, 1890.) 



CONTENTS. 



Page. 



Introduction ' 302 



Physiography of the Piedmont Plateau in Maryland 302 



Topographical and geological Limits 302 



Drainage 303 



Geological Subdivisions 304 



Sources of Difficulty in deciphering the Geology of the Piedmont Area 304 



Primary Sources 304 



Metamorphism 304 



SuperMcial Decay 305 



Cultivation . 305 



Petrography of the Piedmont Region in Maryland 305 



Rocks of the Western or Serai-Crystalline Area 305 



Sedimentary Series 305 



Phyllite 305 



Sandstone 306 



Marble 307 



Eruptive Rocks 307 



Rocks of the Eastern or Holocrystalline Area 307 



Sedimentary Series 307 



Gneiss 307 



Quartzite 308 



Marble (Dolomite) 309 



Eruptive Rocks 310 



Structure of the Piedmont Plateau in Maryland 311 



Description of Sections across the Region 311 



Contrast in Character of the Western and Eastern Areas 313 



Interpretation of the Structure of the Piedmont Region in Maryland 315 



Discussion 317 



A Geological Section across the Piedmont Plateau in Maryland 319 



XLV— Bull. Geol. Soc. Am., Vol. 2, 1890. {^^^} 



