836 E. W. BERRY AGE OF THE MORRISON FORMATION 



deoldea, the at present most (listiiietive feature of the former being the 

 profuse development of the ramentum, doubtless to be correlated with 

 some local climatic variation, and scarcely of generic magnitude. 



The most important paleobotanie evidence bearing on the age of the 

 Morrison formation is to be derived from a consideration of the floras of 

 the Potomac group of the Atlantic Coastal Plain and similar Lower Cre- 

 taceous floras in other areas. That the composition and age of these 

 floras has an important bearing on the age of the Morrison will, I think, 

 be admitted after the relation of the fauna of the Morrison formation 

 and the flora of the Kootenai formation to the fauna of the Arundel for- 

 mation and the flora of the Patuxent and Arundel fornuitions has been 

 discussed. 



Age of the Potomac CT^orp 



Before making these comparisons it will be necessaiy to recall the evi- 

 dence for the Lower Cretaceous age of the various formations of the 

 Potomac group, since this also furnishes a strong argument for the 

 Lower Cretaceous age of the Wealden, at least for the Lower Cretaceous 

 age of the knoA\Ti Wealden floras. 



The Cretaceous age of the Potomac group is indicated 



(1 ) P>y the absence of any nuirine Jurassic deposits in Xorth America 

 east of the Mississippi Piver and the consequent imjjlication that tbi-^ 

 eastern area was above sealevel during all of the Jurassic period. 



{'I) By the presence of a wide-spread peneplain (Weverton), indi- 

 cating a long jjeriod of erosion, during which tbc eastern TJnited States 

 approached baselevel and on the tilted surface of which the sediments of 

 the Potomac grouj) wei"e deposited. 



(3) By the evidence of deep weathering of tlie crystalline rocks which 

 supplied the materials of the Potomac formations. 



(4) By the demonstrated synchroneity of the older Potomac floras — 

 that is, of the Patuxent and Arundel formations — both individually and 

 as a imit, with floras elsewhere, notably in easteni Asia, in South Amer- 

 ica, and in Portugal, where the floras are intercalated in a marine series 

 of more or less abundantly fossiliferous deposits whose age is unques- 

 tionable. 



The details on which this last assertion of synchroneity is based need 

 not be repeated in the present connection, since they have been recently 

 given. ^ It will suffice for the present discussion to say that it rests on 

 all the facts entering into the question of correlation. Each sliows essen- 



" E. W. Berry : liOwer fretaceous floras of the world and correlation of the Potomac 

 formations. Maiyland Oeol. Survey, Lower Cretaceous. PJll. \)\). !)0 172. 



