402 MESOZOIC FLORAS OF UNITED STATES. 



affinities," are greatly modified. A casual comparison of a true Jurassic 

 flora with the Potomac flora shows how profound the modification has 

 been. The Shasta and Kootanie floras show an even more marked 

 Jurassic facies than that of the Potomac, and yet the former of these 

 is proved to be Cretaceous by its fauna, which is abundant, while no 

 one has ever thought of referring the Kootanie to the Jurassic. It is 

 even doubtful whether the oldest Potomac beds are as early as the 

 Wealden. The Wealden of Europe has yielded a large flora, both in 

 England and on the Continent, and yet there has never been found in 

 it anywhere a single even archaic dicotyledonous plant. The evidence 

 of the Cretaceous age for the entire Potomac formation would therefore 

 seem to be conclusive. 



That the Aquia Creek series, or Brooke formation, in Virginia, is 

 largelj^ made up of the materials of the older beds eroded out of them 

 and redeposited has been held by Pi'ofessor Fontaine, and was clearly 

 set forth in my paper on the Potomac formation (p. 326). If the Patapsco 

 and Brooke formations are the same, as there is everj^ reason to believe, 

 the former should have been formed in the same way, and that this was 

 the case is clearlj^ shown in the paper now under consideration. The 

 excellent discussion (pp. 482-483) of the origin of the Arundel clays 

 applies equally to the clay lenses of the James River and Rappahannock 

 series, which are the true homologue in Virginia of the Arundel formation 

 in Maryland. The latter also often forms the base of the Potomac. 



Returning from this survey of the literature to a consideration of 

 the work of collecting and determining the plants of the Potomac for- 

 mation we have to record that on December 11, 1897, all the undeter- 

 mined Potomac material (exclusive of cycads) that had resulted from 

 the field explorations of several j^ears was sent to Professor Fontaine for 

 elaboration, with a view to publication in the series of papers that I was 

 already planning on the Status of the Mesozoic Floras of the United 

 States, but owing to the large amount of work that he had to do on the 

 Triassic and Jurassic floras for the first of these papers slow progress had 

 been made with the Potomac material. On October 8, 1900, a short 

 time before my return from Europe, Prof. W. B. Clark wrote me with 

 regard to the elaboration of the large collections that had then been 

 made, chiefly by Mr. Bibbins, of fossil plants from the Potomac of Mary- 

 land. As these collections were much more extensive than those made 



