FLORA OF OLDER POTOMAC FORMATION. 393 



David White and myself, that the Older Potomac actually occurs in 

 Pennsylvania. 



The year 1896 was the most prolific thus far in the public discussion 

 of the nature and age of the Potomac formation. The Fifteenth Annual 

 Report of the United States Geological Survey contains my paper on the 

 Potomac formation," completed and submitted in June, 1894. It was 

 soon followed by the Sixteenth Annual Report, Part I of which contains 

 Professor Marsh's elaborate memoir on the dinosaurs of North America,'' 

 in which the Potomac vertebrates are described and figured; and my 

 paper'' comparing the Lower Cretaceous of America, and especially the 

 Potomac formation, with the Wealden of England, the Scaly Clays of 

 Ital}^, and the Mesozoic plant-bearing deposits of Portugal. This volume 

 was in the hands of the geologists in October. Professor Fontaine's 

 long-delayed work on the stratigraphical relations of the Potomac 

 formation'' (see p. 358) appeared in December. It had undergone 

 extensive revision at Professor Fontaine's hands since the manuscript 

 was originally prepared in 1883, being designed as a geological introduc- 

 tion to his monograph of the flora of the Potomac formation, but not 

 used as such. The geological map was prepared under my supervision 

 and extends from Petersburg to Baltimore. In it no attempt is made 

 to subdivide the formation. 



These works, in which the age of the Potomac formation was freely 

 discussed, with wide differences of opinion, led to a controversy in the 

 form of short articles by geologists who had paid more or less attention to 

 the subject. The unqualified assertion of Professor Marsh that the Mary- 

 land dinosaur bed was Jurassic, and his final position that the' entire 

 Potomac formation, including the Amboy clays and the beds on Long 

 Island, Block Island,*" Marthas Vineyard, etc., which I had called the 

 Island series, all belonged to that age, attracted special attention. 



a The Potomac formation, by Lester F. Ward: Fifteenth Ann. Rep. U. S. Geol. Survey, 1895, pp. 307-397, 

 pi. ii-iv. 



''The dinosaurs of North America, by Othniel Charles Marsh: Sixteenth Ann. Rep. U. S. Geo]. Survey, 

 Pt. I, 1896, pp. 133-414, pi. ii-lxxxv. 



'^ Some analogies in the Lower Cretaceous of Europe and America, by Lester F. Ward: Op. cit., pp. 463- 

 542, pi. xcvii-cvii. 



''The Potomac formation in Virginia, by William Morris Fontaine: Bull. U. S. Geol. Survey No. 145, 1896, 

 149 pp., map. 



': The geology of Block Island, by O. C. Marsh : Am. Jour. Sci., 4th ser.. Vol. II, October, 1896, pp. 295-298; 

 November, 1896, pp. 375-377. The Jurassic formation on the Atlantic coast: Ibid., December, 1896, pp. 

 433-447. 



