the Middle Atlantic Slope. 129 



pebbles, which are absent southward nearly to the' James, again 

 appear, and at Pohick creek become an important element in 

 the formation ; while the leaf impressions so abundant to the 

 southward disappear. 



About Washington the formation comprises two members 

 sometimes discriminated with difficulty but apparently uncon- 

 formable, the lower consisting of friable arkose or more hete- 

 rogeneous sandstone, with lenticular partings and intercalated 

 beds of clay and notable accumulations of quarzite pebbles 

 (which in the westernmost outcrops become respectable bowl- 

 ders), and the upper consisting of irregularly and discontinu- 

 ously bedded clays, with occasional intercalated beds of sand 

 and pebbles. The clays comprising the greater part of the 

 upper member, like those intercalated within the lower, are of 

 obscure or inconstant structure, variable in composition, and 

 diverse in color : they are sometimes massive and again finely 

 laminated, now silty and unctuous, again made up of flakes and 

 pellets of kaolin, and elsewhere intermingled with sand and 

 grit ; they are commonly gray, whitish, or lead-colored, but 

 again pure white, blue, black, pink, red, brown, purple, or 

 mottled with part or all these colors, generally in soft and deli- 

 cate tints. Silicified and lignitized wood and even lignitized 

 stumps in situ occur in both members ; but identifiable plant 

 remains are not here found. Both members are locally stained 

 and cemented by ferruginous infiltrations. Isolated outliers, 

 made up principally of well-rounded quartzite and vein-quartz 

 pebbles up to six or eight inches in diameter, crown eminences 

 on both sides of the Potomac river, some of those on the south 

 occurring fully twenty miles west of the continuous formation 

 boundary. In this latitude the formation generally constitutes 

 the surface over a belt fully fifteen miles in with, from the 

 Piedmont gneiss on the west to the overlapping Eocene beds 

 on the east ; but the eastern boundary cannot be accurately 

 drawn by reason of the puzzling graduation into newer de- 

 posits, sometimes filling ravines and sometimes covering con- 

 siderable areas, made up of redeposited Potomac materials. 

 Attention was called to the deposits of the formation at Wash- 

 ington by W. B. Rogers in 1875.* 



Between Washington and Baltimore the upper division of 

 the formation (the "Iron Ore Clays" of Tyson f) forms the 

 surface generally and is extensively exposed in the cuttings on 

 both railways and in the numerous workings from which dis- 

 seminated nodules of carbonate of iron are extracted, and has 

 recently yielded abundant dinosaurian remains ; and at Balti- 



*Proc. Boat. Soc. Nat. Hist., vol. xviii, 1875. 



f 1st Rep. State Agricultural Chemist of Md., 1860, 30, 42. 



Am. Jour. Scl— Third Series, Vol. XXXV, No. 206.— Feb., 1888. 



