REPORT ON THE GEOLOGY OF NORTH AMERICA. 37 



surface of both shores of the Chesapeake Bay south of an irre- 

 gular line drawn from Kent county to a few miles below the 

 city of Washington on the Potomac. Over this great area, 

 which is nearly one hundred miles long from north to south, 

 and more than fifty wide, the tertiary beds are seen under 

 nearly uniform characters in almost every spot where the rivers 

 or ravines have exposed sections. The upper layers are usually 

 arenaceous and repose very generally upon a more argillaceous 

 stratum, often developed as an almost pure lead-coloured clay. 

 Both deposits are highly fossiliferous, and when seen on the 

 side of a river, present, sometimes, little else than a bank of 

 shells and zoophytes, often in a state of fine preservation. 



Some of the most conspicuous localities are on the Chester 

 river, which is about the northern boundary ; also at Easton 

 and Cambridge, all on the eastern shore; and again on the 

 western shore of the bay, especially in St. Mary's county, 

 where many of the fossils of this formation were first discovered. 

 Mr. Conrad describes the fossiliferous mass as extending in the 

 precipitous banks of St. Mary's river nearly a mile. This bed, 

 he says, contains many extinct species; it furnishes a large 

 number of genera, with very few species of each, while the in- 

 dividuals are in the greatest abundance. The bank is elevated 

 perhaps thirty feet in the highest point above tide, and there 

 the stratum of shells rises fifteen feet above the river. Siliceous 

 masses with imbedded shells are numerous, and are used for the 

 foundations of buildings. The inferior stratum of these banks 

 is clay, which appears to contain the same species of shells as 

 the sand above it. On the eastern shore of Maryland in the 

 banks of the Choptank, not far from Easton, Mr. Conrad ob- 

 served the following section. 



1. Diluvium. Feet. 



2. Sand, with Pecten Madisonius and Balanus Proteus almost exclu- 



sively 2 



3. Cythe.rea marylandica, Corhula and Pecten Madisonius, in sand ... 7 



4. Cytkerea marylandica in vast numbers, in sand, with Crassatella 



marylandica in abundance 4 



5. Blue clay, with Perna maxillata. 



Virginia. — Tertiary deposits, apparently of the same middle 

 group, occupy, it is believed, nearly all Virginia east of a few 

 miles below the primary boundary, and are seen to put on all 

 the varieties observable in Maryland, being continuous and 

 identical with those just described as belonging to that region. 

 The average breadth of the deposit in Virginia may be stated, 

 therefore, at about sixty miles, and its length the \\'hole extent 

 of the State, from the Potomac south to the State of North Ca- 



