OLDKR POTOMAC OF VIRGINIA AND MAKVLAND. 577 



age foi- the formation," agreeing with the Xeocomian. This conclusion 

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

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

 Wealden is coi'reet — that is, thai it is the nonmarine e(|uival('nt of the 

 Xeocomian. There has been no evidence sufficient to cau.se a change 

 of this oj)inion. but, on the contrary, a good many facts have come to 

 light that confirm its correctness. That is, there is additional i-ea.son 

 to lliink thai 1 he bdwci- Potoniac has a (loi'a that is e.ssentiallv Wealden, 

 and also that the Wealden is Lower Cretaceous rathei' than .liirassic. 



Profe.s.s()r Marsh held that the Wealden is Jurassic. If that is cor- 

 rect the Lower Potomac is .lura.ssic. Since Monograph X\' was written 

 many discoveries of a flora es.sentially Wealden oi- Lower Potomac in 

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

 the marine -Jurassic conditions, which had nrexiouslv prevailed, were 

 followed by those under which nonmarine sediments were laid down. 

 These ^•aried 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 Juras.sic of Oolitic type, now known to exist in 

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

 The localities ftu-nishing fossils belonging to this flora show that it had 

 a surprisingly witle 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 suj^erposed 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 tliaii Jurassic. But we are not without direct evidence 

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

 farther on. 



".M.iiKicrnipli XV, p. 348. 

 .Mo.N .\i.viii -0') 37 



