A REVIEW OF THE CRETACEOUS FLORA OF NORTH AMERICA. 



§1. — GENERAL REMARKS. 



The formation known under the name of Dakota group is positively 

 determined as Cretaceous by the animal-remains profusely embedded 

 Into the strata overlying it. This fact has been repeatedly and clearly 

 exposed in the former reports of Dr. F. V. Hayden. As this formation 

 rests immediately upon thick limestone beds of Permian age, its flora, 

 which is mostly represented by dicotyledonous leaves, has apparently no 

 ancestors in this country. In Europe, the dicotyledonous plants of the 

 Cretaceous epoch are scarcely known, or, at least, they have not yet 

 been satisfactorily studied and described. The more recent and impor- 

 tant publication on the subject refers to the Cretaceous of Greenland, 

 and exposes the specific characters of a proportionally large number of 

 Cryptogams and Gymnosperms, Ferns, Conifers, Cycads, with few Dicoty- 

 ledonous. Three of these only are represented in the flora of the Dakota 

 group. There is, therefore, from antecedents or from contemporaneous 

 floras, no points of comparison to which the character of the plants of this 

 group might be referred. For analogies, we have to look to spe- 

 cies described from more recent epochs. And, in these researches, 

 the paleontologist is met with another kind of difficulty. The strata 

 where the dicotyledonous leaves are found in Kansas, Nebraska, Dakota, 

 &c, are separated from the Lignitic-Tertiary formations by a few thou- 

 sand feet of measures, mostly shale and sandstone, all of marine origin, 

 with animal fossil-remains denoting an uninterrupted series of Creta- 

 ceous types. These strata are generally overlaid by heavy beach-sand- 

 stone locally interspersed with fucoidal remains, extremely abundant in 

 some places, or with a mass of crushed, half-pulverized fragments of land- 

 plants. Over this, the Lignite beds come to view, with their accom- 

 panying shales and sandstones, wherein vegetable remains are found 

 sometimes in profusion and in a beautiful state of preservation. Here, 

 then, we should expect to recognize forms of leaves or species, if not 

 identical with those of the Dakota group, at least showing, as probable 

 offsprings, some affinity of characters with them. This as yet is not 

 the case. The typical forms of leaves of the North American Creta- 

 ceous are not at all repeated in the Lower Lignitic flora of the Rocky 

 Mountains, not more than they are in the Lower Eocene of Europe. 

 With the exception, however, of the peculiar type of oak and chestnut, 

 Dryophyllum, which originates in the Middle Cretaceous of both con- 

 tinents, is recognized in species of the Lower Eocene of France, Sezaue 

 and Gelinden, as in that of Point of Rocks, in Wyoming, and leaving 

 some of its representatives in all the geological series, passes to the 

 flora of our time. Some few more leaves of the Dakota group have a 

 relation to species of Evanston, especially to those of Miocene of Car- 

 bon, in the same proportion, about, as they have to Miocene species of 

 Europe ; more still are closely allied to species of the Pliocene of Cali- 

 fornia ; but the analogies become far more evident and marked, also, by 

 316 



