Introduction. 



The three subdivisions which McGee has clearly shown to be equally 

 characteristic of both the geology and topography of the middle Atlantic 

 sloped are admirably contrasted by the cross-section made of them by the 

 state of Maryland. The land area of the state may be set down in round 

 numbers as 10,000 square miles. By a similar approximation, the western 

 2,000 square miles of this territory may be assigned to the Ajjpalachian zone ; 

 the central 3,000 to the Piedmont region ; and the eastern 5,000, which sur- 

 round Chesapeake bay, to the coastal plain. 



In the west the Paleozoic strata are thrown into broad and gentle undu- 

 lations, which increase steadily in abruptness as we move eastward. Finally 

 a certain amount of recrystallization accompanies the more intense disturb- 

 ance, and positive evidence of the age of the oldest and most easterly strata 

 seems to have been obliterated. Between the last Paleozoic beds whose 

 exact horizon is fixed by fossils, and the broad transgressions of unconsoli- 

 dated Mesozoic formations which occupy the eastern half of the state, there 

 spreads out in Maryland an expanse of high, but nearly level, country 

 which forms a portion of the Piedmont plateau. This is composed partly of 

 semi-crystalline, partly of highly crystalline rocks, of whose age we have as 

 yet no direct paleontological evidence. The accompanying mapf (plate 12) 

 exhibits the outline and drainage of this area, as well as the approximate 

 line of boundary between its semi-crystalline and holocrystalline portions. 

 The mapping and comparative study of these rocks, while still by no means 

 completed in its details, strongly indicate the entire geological independence 

 of these semi-crystalline and crystalline areas, as it will be the aim of this 

 paper to show. 



Physiography of the Piedmont Plateau in Maryland. 



Topographical and geological Limits. — Topographically, the I iedmont 

 plateau in Maryland may be considered to begin at the eastern base of 

 Catoctin mountain, a sharply defined ridge of nearly uniform height (1,500 

 feet), extending from Point of Hocks, on the Potomac, northward to the 

 Pennsylvania line, just west of Eramitsburg. East of the Catoctin mount- 

 ain nearly three thousand square miles are exposed before the overlap of 

 coastal clays and gravels is encountered. Geologically, however, the western 



*Am. Jour. Science, 3d series, vol. XXXV, 1S88, p. 121; also Seventh Annual Report of the 

 Director of ih^ U. S. Geol. Survey, Wa.shington, 1888, p. 548. 



fThe capital initials upon the map (plate 12) and the sections (figures land 2, p. 312) indicate 

 towns as follows: E, Emmitsburg; F, Frederick ; TF, Westminster; A, Mt. Airy; O, Glencoe; G F, 

 Great Falls. 



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