POTOMAC FORMATION IN VIRGINIA AND MARYLAND. 599 



Bluff none of them j'ield a characteristic Brooke flora, and they are for 

 the most part, so far as at present Icnown, practically barren. I include 

 the Federal Hill beds in the Patapsco, making the upper part the homo- 

 logue of the Mount Vernon beds, but as only 25 feet of the latter have 

 been measured, while at Federal Hill we have an actual section of 46 

 feet," it is assumed that these beds extend some 25 feet lower. Giving 

 the Virginia beds the same thickness as before, the Patuxent with its 

 included Arundel occupy 325 feet, or 25 feet less than the combined 

 James River and Rappahannock, assuming the base of the formation 

 to be the same in both States. 



As thus presented, the evidence derived from the stratigraphy of 

 the Potomac formation harmonizes perfectly with that derived from 

 the fossil plants, and the correlation of the formation in Virginia and 

 Maryland is complete. 



"See The Potomac formation; Fifteenth Ann. Kept. U. S. Geol. Surv., 1895, p. 331. 



