62 THE I'OTOMAO OR YOUNGER MESOZOIG FLORA. 



they were much larger than their representatives and united to form a 

 great estuary whose waters extended at least from Petersburg to Balti- 

 more, we could explain the origin of many of the features of the formation 

 now in question. 



The character and distribution of these beds seem to indicate that the 

 margin of the continent covered by these estuary waters was subjected to 

 oscillations of level, so that the shore-line retreated and advanced. If we 

 assume that this margin was first rapidly depressed and then elevated 

 without pause, unquiet conditions would prevail throughout the epoch, and 

 marine life could not be established in the area affected. 



The tendency of the action of the waves and river currents would be 

 to work over deposits recently made and to produce the peculiar effects 

 due to alternating erosion and sedimentation which are so conspicuous in 

 the lower Potomac beds. The tendency would also be to work the 

 deposited matter seawards and to give in that quarter thicker accumula- 

 tions than those found on the western margin, for this would be submerged 

 only temporarily. 



If these assumptions be correct, thei'e would be a noteworthy simi- 

 larity in tlie conditions attending the formation of the lower Potomac 

 strata and those of the Wealden of Europe, and it is interesting to note 

 thai in the plant remains of tlie lower Potomac there is an important ele- 

 ment which finds its nearest relations in that formation. 



