OLDER POTOMAC OF VIRGINIA AND MARYLAND. 577 



age for the formation," agreeing with the Neocomian. This conclusion 

 was based on the strong affinity of its flora with that of the Wealden. 

 It was assumed that the view generally held as to the position of the 

 Wealden is correct — that is, that it is the nonmarine equivalent of the 

 Neocomian. There has been no evidence sufficient to cause a change 

 of this opinion, but, on the contrary, a good many facts have come to 

 light that confirm its correctness. That is, there is additional reason 

 to think that the Lower Potomac has a flora that is essentially Wealden, 

 and also that the Wealden is Lower Cretaceous rather than Jurassic. 



Professor Marsh held that the Wealden is Jurassic. If that is cor- 

 rect the Lower Potomac is Jurassic. Since Monograph XV was written 

 many discoveries of a flora essentially Wealden or Lower Potomac in 

 character have been made, which indicate that it existed at a time when 

 the marine Jurassic conditions, which had previously prevailed, were 

 followed b}' those under which nonmarine sediments were laid down. 

 These varied a good deal. Sometimes the sea was advancing over the 

 land, and shallow water, estuarine, and other similar sediments accumu- 

 lated. In other cases lakes and marshes were formed, sometimes with 

 the accumulation of vegetable matter for the formation of coal beds. In 

 these deposits the Lower Potomac flora is found. This flora is a transi- 

 tion one from the typical Jurassic of Oolitic type, now known to exist in 

 Oregon, to the typical Cretaceous of the Dakota and later formations. 

 The localities furnishing fossils belonging to this flora show that it had 

 a surprisingly wide distribution. It has been found as far north as the 

 Kootanie in British Columbia and as far south as Tlaxiaco in Mexico. 

 On the eastern side of the continent this fossil flora exists in the eastern 

 portions of Virginia and Maryland. On the western side it is found in 

 the Shasta group of California. The fact that this flora is found in strata 

 which are often superposed on Jurassic beds with want of conformity, 

 and which by their character show that important changes had taken 

 place since the Jurassic conditions prevailed, and the farther fact that 

 the flora is transitional, make it a priori probable that it is Lower Creta- 

 ceous rather than Jurassic. But we are not without direct evidence 

 that this flora is Neocomian or Lower Cretaceous. This will be given 

 farther on. 



a Monograph XV, p. 348. 

 HON XLVIII — 05 37 



