XXX INTRODUCTION. 
Fontaine’s figures, pl. lxxxii. and Pl. 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 Quadersandstein, also with the 
floras of NiederschOna and Quedlinburg.! 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 Glerchenia 
Nordenskisldi, Heer, and Sequoia Reichenbachi, 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.’ 
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.» 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 im which they 
are found.’’+ 
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. 
* Ibid. pp. 222 et seg. ‘‘ Table of distribution.” 
3 Bull. U.S. Geol. Surv. No. 82, 1891. 
4 Ibid. p. 90. 
