FLORA OF OLDER POTOMAC FORMATION. 395 



Professor Marsh's general claim that the Wealden should be referred to 

 down the Jurassic. 



In order still further to emphasize the wide difference between the 

 Older and Newer Potomac, and also to give the views of Professor Fontaine, 

 who had most full}^ studied the former, and of Doctor Newberry, who was 

 at the time of his death the first authority on the latter, I made a second 

 contril:)ution " in the spring of 1897, quoting somewhat extensive^ from 

 those authors, and endeavoring to show that Doctor Newberry placed 

 the Amboy clays somewhat too high, while Professor Marsh placed them 

 much too low and confounded them with the Older Potomac. 



Professor Clark and Mr. Bibbins published in August, 1897, '' a some- 

 what full account of the results at which they had arrived in their studj^ 

 and preliminary survej^ of the Potomac formation in Maryland. They 

 admit the great difference between the age of the lower and the upper 

 beds, and sustain the view which I maintained in my paper on the Potomac 

 formation in 1895, that it consists of a series of beds dipping coastward and 

 beveled on the surface, so that in crossing the belt from northwest to 

 southeast one rises in the geological scale from the lowest to the highest 

 beds ; in other words, that the Potomac formation is not a ' ' trough, ' ' as 

 was formerly supposed, but an integral part of the sedimentary beds that 

 make up the coastal plain. They did not, however, accept the nomen- 

 clature that I proposed, but adopted an entirely different one, making four 

 instead of six subdivisions, which in ascending order are as follows: 

 Patuxent, Arundel, Patapsco, Raritan. On page 481 they say: 



It is the conclusion of the authors, founded upon a detailed stratigraphic study 

 of the Potomac group, that all the beds which have afforded dicotyledonous types of 

 plant life are above those which have jaelded the vertebrate remains, and, moreover, 

 that a marked unconformity exists between the two series of deposits. The evidence 

 for this conclusion will be brought out in the succeeding pages. 



This was an inference only, and has been disproved by the study of 

 the plants that had been alread}^ collected. The Patuxent formation is 

 described as follows: 



The deposits of the Patuxent formation consist mainly of sand, at times quite 

 pure and gritty, but generally containing a considerable amount of kaolinized feld- 



« Professor Fontaine and Doctor Newberr_v on the age of the Potomac formation: Ibid., March 12, 

 1897, pp. 411-423. 



'' The stratigraphy of the Potomac group in Maryland, by Wra. Bullock Clark and Arthur Bibbins: Journ. 

 Geol., Vol. V, No. 5, July^August, 1897, pp. 479-506. 



