I'OTOMAC FORMATION" IN V]R(;iNIA AND MARYLAND. 599 



Bluft" none of tli(Mii yield a characteristic Brooke flora, and tliey are for 

 the most part, so far as at present known, pi'actically l)ari-en. I inchide 

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

 logue of the Mount Vernon t)ed.s, l)ut 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 l2o feet lower. (living 

 the \'irginia l)eds the .same thickness as before, the Patuxent with its 

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

 James River and Rappahannock, assuming the l)ase of the foi-mation 

 to be the same in both States. 



As thus presented, the evidence derived from the stratigraphy of 

 the Potomac formation harmonizes perfectly with tliat derived from 

 the fossil plants, and the correlation of the formation in ^'irginia and 

 Maryland is complete. 



"See The Potomac formation; Fifteentli Ami. Kept. U. S. Geol. Surv., 1S9.5. p. 331. 



