194 TKANSACTIONS OF THE [1892 



They do not agree in composition and structure with, the 

 hollow or lens in the streets of Fredericksburg, from 

 which Prof. Fontaine and myself excavated so many fossil 

 leaves, twigs, etc. 



The Fredericksburg deposit is, to my view, a structure 

 built at a much later date than the Falmouth clays, and the 

 series of strata to which it belongs has been built within ,an 

 eroded area. The sandstone member of the Acquia creek 

 region, as seen below Fredericksburg and everywhere else 

 in Maryland and Virginia, is a whole formation higher 

 than the aforesaid clays. 



Potomac Formation. — In Maryland this series of beds 

 rests partly on the degraded surface of the crystalline and 

 Archaean rocks. The lowest members thus far observed 

 are dark lead-colored, drab or pale clays, with intervals of 

 chiefly sharp sand, resting in places upon a basal bed of 

 broken micaceous material, often disintegrated into hetero- 

 geneous masses or layers, but in other places largely made 

 up of the comminuted or broken fragments of the hard 

 rocks of the vicinity. Upon this rests a bed of sand or sandy 

 clay of variable thickness, followed next above by layers of 

 dark clay which were invaded by cross currents so as to 

 break their continuity throughout a distance of five or more 

 miles. 



In Maryland, and particularly in Baltimore and its 

 vicinity, we find a characteristic coarse, angular, whitish 

 sand, sometimes discolored by iron solutions, which rises 

 from beneath, or not far above, tide level, invaded by lumps 

 of kaolin. This sand is usually bounded above by a nar- 

 row layer of dense iron stone, varying from two to about 

 six inches in thickness. Directly resting upon this is a 

 distinctly stratified group of dark and drab clay beds, hav- 

 ing a total thickness of three to seven feet, parted by sandy 

 layers or by slickenside surfaces at the lines of contact. In 

 these clays, which are in place and continuous through long 



