GEOLOGICAL POSITION OF THE LOWER POTOMAC BEDS. 59 



enough must liave elapsed to ponnit an almost total change in the geo- 

 logic conditions under which the two formations were laid down. The 

 inland bodies of water, in which the older Mesozoic was for the most part 

 deposited, had been drained off, so that deposition in Potomac times took 

 place only outside of the crystalline terrane, within which nearh' all of the 

 older Mesozoic is to be found. The older Mesozoic areas and the crystal- 

 line rocks adjoining them are cut by numerous trap dikes. Not a trace of 

 these traps can be found in the Potomac beds. The period of igneous 

 activity which has left so many traces, probably at the end of the Older 

 Mesozoic time, must have then been over when the Potomac sediments 

 were accumulating. These facts do not enable us to fix the lower limit of 

 the Potomac horizon very definitely, but at least we can say that the forma- 

 tion is considerably younger than the Rluetic. 



The upper limit in age can be fixed more nearl)'. The marine Cre- 

 taceous of New Jersey and Maryland is wanting in Virginia, as is the 

 Variegated Clay formation. In this State the oldest formation which rests 

 upon the Potomac, and whose age is fixed, is the Eocene. Well-exposed 

 contacts of the Eocene with the older formation may be seen in a number 

 of places. The greensand marl bed is the oldest portion of the I^ocene that 

 contains an)- considerable number of fossils. This bed contains, among 

 others, such shells as Osfira, TitrrUcUa, Pectcn, Ci/tlirred, ('iicullca. This bed 

 in all of the area south of Fredericksburg, and at this place also, rests 

 directly upon the tyi)ical sand of the lower Potomac. This latter, in all 

 such cases, gives evidence of having lost much from erosion, and its up[)er- 

 most portion nearly always contains large cobbles. Towards the northern 

 part of tlie State we find under the greensand bed, it is true, a variable 

 thickness, amounting in some cases to from seventy to ninety feet, of non- 

 fossiliferous sandy matter. This, however, from the glauconite that it con- 

 tains and other features, belongs to the Eocene formation. Under it the 

 lower Potomac still shows marks of great erosion. 



The Eocene is a true marine formation, laid down under very difterent 

 conditions from tliose attending the deposition of the lower Potomac. The 

 eastern margin of the continent must have become depressed enough to 

 permit the sea to occupy portions that in lower Potomac times showed 



