6o 



Cincinnati Society of Natural History. 



structure of the Piedmont plateau, of which this area is a 

 portion, has been described by Williams,* and the topography 

 has been outlined by McGee..f 



ROCK EXPOSURES. 



The uneven surface of the region is very favorable for good 

 rock exposures, which are seen in the quarries, along the 

 streams, and where the various roads cut into the hills as they 

 descend into the valleys. In the latter class of exposures, 

 however, the granites are often so much weathered that the 

 specimens are not typical. Better exposures are to be seen 

 along the streams. The banks of the Susquehanna River, 

 within the area studied, present three different types of expo- 

 sure. North of Rowlandville, the main granite wall, which 

 is less than one hundred feet in height, stands back from the 

 river two hundred feet or more and slopes down to the stream, 

 so that the country rock is concealed to a considerable extent 

 by vegetation and rock decay. In the space between this 

 cliff and the river are large gabbro blocks which have been 

 washed down from the north, probably in Columbia time. 

 Below Rowlandville, for a distance of two miles, the rock 

 w T all retreats so far and becomes so low that no good expo- 

 sures are afforded. At a distance of one mile above Port 

 Deposit, the rock wall becomes perpendicular and approaches 

 close to the river. This is continuous to Perryville, a dis- 

 tance of six miles, with an average height of nearly one 

 hundred feet. In the vicinity of Port Deposit the natural 

 river wall has been artificially moved back, to secure addi- 

 tional space for the town and railroad, and this has rendered 

 the exposures especially fine. One mile south of the town 

 the granite wall is cut by a number of diorite dykes, which 

 vary in width from a few inches to five hundred feet. The 

 granite, at this point, stands nearly vertical, and is much 

 squeezed and crushed, having developed in it large eyes or 

 nodules of epidosite. Through weathering, these cliffs pre- 

 sent, especially toward the top, a very irregular, jagged 



*Bul. Geol. Soc. Amer., Vol II, pp. 303-318, 1890. 



t Amer. Jour. Sci. 3 Vol. XXXV, p. 121, 1888 ; Annual Report U. S. Geol. Survey , 

 Vol. VII, p. 616, 1S88. 



