GEOLOGICAL AFFINITIES OF THE POTOMAC PLANTS. 333 



difference is much greater than one wouhl expect in the fossils of two for- 

 mations so near each other in geological age. 



All of the numerous remarkable types so characteristic of the Potomac 

 strata are wanting in the New Jersey Cretaceous. 13ut besides this, in tlie 

 New Jersey beds angiosperms constitute much the greater part of the plants. 

 In the Potomac formation the ferns, cycads, and conifers form much the 

 greater element in the flora. One is induced to think that in producing 

 such an entire cliange in the vegetation some other cause besides the mere 

 lapse of time must have acted. The alteration is mainly caused by the 

 very general extinction of the Jurassic types which still survived in the 

 Potomac flora, and of the warmth-loving plants which existed in it. The 

 question might be asked, Was this change hastened by the coming on of a 

 cold period in this region in the interval between the lower Potomac and 

 the New Jersey Cretaceous ? 



GEOLOGICAL AFFINITIES OF THE POTOMAC PLANTS. 



In the description of the different species of Potomac jilants attention 

 was called in each case to the known fossils that are identical with or most 

 neai'ly allied to the plant being described. It will perhaps be useful to re- 

 peat in this place these statements, in part at least, and to add such details as 

 may suggest themselves, in order that we may get a connected view of the 

 geological relations of the flora. For this purpose I will take up in succes- 

 sion the different groups of plants, and try to give what may be deduced 

 from their character. 



It might cause error to attempt to determine the age of the flora ])y 

 giving percentages of the elements common to the Potomac and known 

 floras. In many cases particular species are represented by few individu- 

 als, and these are not well characterized. In other cases, much fewer in 

 number perhaps, the species may be represented by numerous individuals 

 which are well characterized and widely diffused. Species of the kind last 

 mentioned, although much inferior in numbers to the kind first named, ought 

 to have more weight than these in fixing the character and determining the 

 age of a flora. Then, again, some species may be merely survivors of an 

 older flora, or newly introduced precursors or prototypes of forms that 

 attain their typical character later. These are not of so much value in 



