XXX . INTRODUCTION-. 



Fontaine's figures, pi. Ixxxii. and PI. VII. Fig. 3 of the present 

 Catalogue. 



In a posthumous work by Lesquereux on the Flora of the 

 Dakota group, edited by Knowlton, we find this American flora 

 correlated with that of the Harz Quadersand stein, also with the 

 floras of Niederschona and Quedlinburg. 1 Reference has been 

 made to these German floras because certain of their floral elements 

 show a close relationship to members of the typical Wealden 

 vegetation ; in the Dakota flora there seems to be hardly the 

 same reason for comparison with the Wealden floras of Southern 

 England and Northern Germany. The two species Gleichenia 

 Nordenskioldiy Heer, and Sequoia JReichenbachi, Gein., are common 

 to the Dakota and Potomac floras ; several species also occur in the 

 Kootanie flora, in the Neocomian of Westphalia and the Urgonian 

 of Kome, but we do not find a distinct Wealden facies in the 

 Cenomanian Dakota flora. 2 



For a critical account of the Cretaceous rocks of America 

 reference should be made to the Cretaceous Correlation papers 

 by C. A. White, which have appeared in a recent number of 

 the United States Geological Survey Bulletins. 3 The Potomac 

 formation is provisionally assigned to the base of the Cretaceous 

 system, but stress is laid on the difficulty of arriving at any very 

 definite conclusions as to the real age of this widespread deposit. 

 It is pointed out that a large proportion of the plant remains 

 figured by Fontaine from Virginia were found in rounded and 

 lenticular masses of indurated clay imbedded in the Sandstone or 

 Arkose deposit: "One is therefore disposed," says White, "to 

 inquire whether the plants may not represent a somewhat older 

 deposit than is that part of the Potomac formation in which they 

 are found." 4 



No attempt is made in this Correlation paper to correlate the 

 American divisions of the Cretaceous system with their European 



1 Dakota Flora, p. 20. 



2 Ibid. pp. 222 et seq. " Table of distribution.' 



3 Bull. U.S. Geol. Surv. No. 82, 1891. 

 * Ibid. p. 90. 



