20 THE FLORA OF THE DAKOTA GROUP. 



Minnesota, now in progress, will undoubtedly clear up much that still 

 renmins uncei'taiu concerning the width of the area occu})ied by the Dakota 

 Group in the United States, the thickness of its deposits, the composition of 

 the strata observable at different localities, as well as the direction and 

 degree of the dip, etc. 



The present memoir is for this reason limited to the description of fossil 

 plants represented by a large number of specimens recently obtained at 

 different localities of the Dakota Group, especially in Kansas and, of course, 

 to the evidence derived from the character of the plants in regard to their 

 origin, their relations, and their places in the history of the vegetation of 

 the world. 



The significance attached to the nature of these plants is well known. 

 They pertain to an epoch in which, by the appearance of the dicotyledons, 

 the character of the flora of the globe has been modified as though by a 

 new creation. The cause or reason of this marked change remains still 

 unexplained, and can become known only by a more intimate acquaintance 

 with the flora of that part of the Middle Cretaceous which is generally 

 recognized as the Cenomanian period. This flora is known in Europe by 

 remains of plants found in the Quadersandstein of the Harz, and first 

 described by Hampe, later by Zenker, Dunker, and Stiehler, and represent- 

 ing twenty-five species; then by those discovered in the Cretaceous strata 

 of Niederschona, Saxony, from which Ettingshausen has descriljed thirty 

 species; then by sixteen species described by Heer from Moletein, in 

 Moravia; by sixteen described by the same author from Quedlinburg, 

 Prussian Saxony, and by seventy-five species from the Bohemian Cretaceous 

 described by Velenovsky. All the localities named above are far distant 

 from each other, but have been with more or less doubt refen-ed to the same 

 horizon of the Middle Cretaceous, \'iz, the Cenomanian. Admitting the 

 correctness of tlie reference, we have in all about one hundred and ten 

 species as constituting the flora of the Cenomanian of Europe. This seems 

 a small number indeed, for two hundred and seventy-four species have been 

 described by Heer from the Cenomanian of Greenland, to which must now 

 be added the plants from the Dakota Group, from which four hundred and 

 sixty species are known. 



In my Cretaceous Flora the questions concerning the probable 

 derivation of the numerous vegetable remains found in the shaly sandstone 

 of the Dakota Group, their mode of deposition, etc., have been examined. 

 From the facies and the peculiar distribution of the leaves, it is there 



