FLORA OF OLDER POTOMAC FORMATION. 351 



Their age has been hitherto quite uncertain; they have been stated by Meek and 

 Hayden to be the eariier division of tlie later Cretaceous of the general geologic 

 series. They extend across the States of Delaware, Maryland, and Virginia. In 

 Marj^land they are stated by Ducatel to contain the important deposits of carbonate 

 of iron; and Philip Tyson, State geologist, informs me that these beds lie upon the 

 red and blue clays, forming hills, which have been produced by erosion of the valleys 

 to the beds below. These iron clays contain several species of cycadaceous plants, 

 whence Tyson infers the age of the clays to be Jurassic and not Cretaceous. 



There are in the museum of the Smithsonian Institution, Washington, several 

 specimens of fossil Unios, from a ferruginous clay which crops out at some elevation 

 on the banks of the Potomac. These species are identical with those which have 

 been found in the New Jersey clays, and the deposit is doubtless the same as that 

 which traverses the State of Marjdand. 



Indurated grey clays on the Rappahannock River have been examined by my 

 friend Philip R. Uhler, of Baltimore, who has obtained from them leaves and stems 

 of some six species of plants, in beautiful preservation, of the orders Cycadacese,? 

 Gnetaceae and Filices. The position and character of this bed render it excedingly 

 probable that it is a contmuation of those of Maryland and Alexandria. 



The whole formation indicates the existence of an extended body of fresh water, 

 having a direction and outline similar to that in which were deposited the red 

 sandstones and shales of the Triassic belt, which extends parallel to its northwest 

 margin throughout the States in which it occurs, separated, except in New Jersey, 

 by a broad band of gneiss and Potsdam rocks. The carbonate of iron was no 

 doubt deposited in a bog or bogs along its margin or in its shallows, as the bottom 

 became elevated, as suggested by Tyson, though not in a salt-water swamp, as 

 supposed by him. The Cycads and dicotyledonous trees grew in the swamps and 

 on the shores, while terrestrial reptiles of large size no doubt haunted their shades. 



These beds appear to dip conformably beneath the Lower Cretaceous marine 

 beds in New Jersey, in which, at a distance of a few miles from their border, occurred 

 the remains of the Hadrosaurus; and it is therefore not probable that they were 

 cotemporary with these, as is the case with the Wealden of Kent and the Creta- 

 ceous at Maidstone, England. The Hadrosaurus clays, belonging to the Upper 

 Cretaceous, as indicated by the presence of many molluscs of the Ripley group of 

 Mississippi, appear to be separated from the clays in question by a great lapse of 

 time. The age is therefore probably truly Wealden or Neocomian. 



These facts indicate the existence of a barrier to the eastward of their present 

 position, which for a long period prevented the access of salt water. This barrier 

 was no doubt an anticlinal of the Appalachian series, outside of that which walled in 

 the Triassic fresh-water area, and, like it, parallel with the general series of anti- 

 clinals of the present Allegheny range. That it was, like the latter, at one time 

 submarme, and, gradually rising, finally enclosed the area in question, the waters 

 of which soon became fresh, from the numerous rivers which flowed into it. 



