350 CONTRIBUTIONS TO THE GEOLOGY OF 



yond them to the east. It was found, however, that strata of clay, 

 lying adjacent to the eocene on this side, occupied an interval in which 

 fossils of neither of these formations could be distinctly found; and 

 that still further on, the beds of the meiocene came in view. This 

 intervening tract, as seen upon the Potomac and Rappahannock, is flat 

 and low. 



Without, then, pretending to an exact delineation of the boundary 

 in question, which, from the natureof the case, would be impracticable, 

 it will be sufficient for the present to consider it as coincident with a 

 right line, connecting the mouth of Chingoteague creek on the Rap- 

 pahannock, with Mathias's Point upon the Potomac. A brief account 

 of the character and situation of the strata, as observed at these two 

 points, will serve to illustrate the propriety of fixing upon them as its 

 termination. 



To the WTst of the mouth of Chingoteague creek, for a distance of 

 more than half a mile, the north bank of the Rappahannock has an 

 average height of about fifteen feet above the river. At its upper end 

 this bank consists of a stratum of the green sand marl, extending to 

 the height of twelve feet above the water line, upon which reposes a 

 layer of diluvial sand and clay, about three feet in thickness. In ap- 

 proaching the creek, the level of the marl stratum is observed steadily 

 to decline, while the thickness of the incumbent bed augments until, 

 at a point within two hundred yards of the mouth of the creek, the 

 former entirely disappears below the level of the river. At this point 

 the diluvial capping is about fourteen feet in thickness, consisting of a 

 layer of sand and pebbles about seven feet thick, resting upon a stra- 

 tum of whitish clay, which reaches to the water line. 



The eocene character of these subjacent beds is unequivocally marked. 

 At the base of the bank a dark greenish layer presents itself, rising to 

 the height of five or six feet, containing numerous impressions of 

 the eocene Carditas, and other shells of a brownish colour. Over this 

 is a layer of a lighter hue, containing bands of white calcareous matter, 

 obviously the remains of shells. 



To the east of Chingoteague creek the bank preserves its former 

 height for about three-fourths of a mile ; after this it becomes much 

 depressed, and continues to be low for some distance down the river. 



