444 N. H. BARTON — LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



lu Virginia, southward from Fredericksburg, it extends far westward on 

 all of the divides, but with gentle seaward inclination follows down the 

 drainage eastward. The diatomaceous deposits are variable in size and 

 purity, and are irregularly scattered through the clays without restriction to 

 any definite stratum. 



The medial clays and marls are relatively thin, but they occupy a con- 

 siderable area in Virginia. The finest exposures are on the James river 

 near Claremont, near Hanover Court-House, in the Bowling Green region, 

 in Nomini cliffs on the Potomac river, on the lower St. Mary's river, and 

 near West Point on the York river. 



A well-known exposure of the upper beds of the Chesapeake formation is 

 found in the cliflfs at Yorktown ; and other fine exposures are at Grove 

 wharf, Smithfield and Claremont on James river, at Suffolk on Nansemond 

 river, at Lanexa on the Chickahominy, at Urbana on the Rappahannock, 

 along the lower Patuxent river and the adjoining shores of Chesapeake bay, 

 and near Easton as well as elsewhere on the Choptank river. At all these 

 points the Yorktown fauna is well represented, and the remains occur in 

 great abundance, Claremont on James river being an especially noteworthy 

 locality, although less known than some of the others. 



Stratigraphie Relations. — For the greater part of its area, the clays of the 

 Chesapeake formation lie directly on the eroded surface of the Pamunkey 

 greensands. Westward at some points it overlaps for short distances on the 

 Potomac formation and crystalline rocks. On James river below City 

 Point the medial portion of the formation lies on Pamunkey greensands, 

 indicating an island or local shore bluff* in the early Chesapeake seas. Else- 

 where the stratigraphie position of the base of the formation appears to be 

 constant, and the basal plane is a smooth surface inclined eastward very 

 uniformly at the rate of about ten feet to the mile. 



In the Washington section the base of the Chesapeake formation locally 

 cuts across the thin edges of the Pamunkey and Severn formations, and lies 

 directly on the Potomac formation. At Good Hope hill, in this region, occur 

 the Eocene fossils mentioned by McGee,* but they are found to be casts 

 mixed with casts of Cretaceous species, both imbedded in sands containing 

 impressions of Miocene moUusca. This occurrence of pebbles, in part con- 

 sisting of fossil casts, is quite common at the base of the Chesapeake forma- 

 tion, notably at Herring bay and on the Pamunkey river. In Maryland, 

 especially near Nottingham and on Pope's creek, the base of the formation 

 consists locally of a thin, hard silicified stratum filled with Miocene mol- 

 luscan impressions. 



The Chesapeake formation is unconformably overlain by the Appomattox 

 formation, and along the bay shores and stream depressions by the Columbia 

 formation. 



* " Three Formations of the Middle Atlantic Slope : " Am. Jour. Sci., 3d sen, vol. XXXV, p. 136. 



