FLORA OF OLOFH POTOMAC FOKMATION. 851 



Their age has been liitlici'tu (|uitc uticcrtaiii; llicy liavc hccii stated bv MccU and 

 Harden to be llic earlier division of tlie hiter Cretaeeous of the genera! ge(dogie 

 series. They ext.end across tiie States of Dehiw are, Maiviand, and Virginia. In 

 Mar^'land they are stated by Dueatel to eoiitain llic iiii|i()rlanl deposits of carbonate 

 of iron; and l*hiii|) Tyson, .State geologist, informs me ib.-ii these beds lie upon the 

 red and blue clays, forming hills, which liaA'c been pi-o<luced 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 cliiys to be .liirassic and not Cretaceous. 



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

 s])ecimens of fossil I'nios, from a ferruginous clay which crops out at some elevation 

 on th(> banks of the Potomac. The.se species are identical with those which liave 

 been found in the New .h'rsey clays, and the deposit is doubtless the same as that 

 which traveises the State of Maryland. 



Indurated grey clays on the Kii])pahannock Iviver have bi>en examined by my 

 friend Philip IC Chler, of Ualtimore, who has obtained IVoiii them leaves and stems 

 of some six species of plants, in beautiful ])reservation. of the orders Cycadace*,? 

 Gnetacea? and Filices. The i)osition and character of this bed render it excedin^ly 

 probable that it is a continuation of those of Maryland and Alexantlria. 



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 Tria.ssic belt, which extends parallel to its northwest 

 margin thi-oughout- the States in which it occurs, separated; except in New Jersey, 

 by a broad band of gneiss and Potsdam rocks. The cai-bonate of iron was no 

 doubt deposited in a bog or bogs along its nuirgin 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 gi-ew in the swam))s and 

 on the shores, while terrestrial reptiles of large size no doubt haunted tlieir 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, a|)pear 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 (he access of salt water. This barrier 

 was no doubt an anticlinal of the Appalachian s(>ries, outside of that which walled in 

 the Triassic fresh-water area, and, like it, i)arallel with the general series of anti- 

 clinals of the present Allegheny range. 'I'hat it was, like the latter, at one time 

 submarine, and, gradually rising, finally enclosed the area in cjuestion, the waters 

 of which soon became fresh, from the mim(>rous rivers which flowed into it. 



