243 



the species of trees and shrubs which have at the present time the more 

 general and the widest range of distribution. Indeed, most of the 

 genera of the arborescent North American flora are represented in the 

 Cretaceous by analogous types, with the exception of those which are 

 characterized by serrate, denticulate, or crenate leaves, like Tilia, 

 JEsGulus, the serrate Bosacece, Hamamelis, Fnixlnus, the Urticinece, like 

 Flanera^ Ulmus, and of the Amentacece, Betula, Almis, Garpinus, Goryliis, 

 Carya, etc., all with serrate or dentate leaves. The more appreciable 

 and general characters of the Cretaceous leaves are a generally thick 

 coriaceous substance and the integrity of the borders. From this it is 

 possible to derive some reliable conclusions in regard to the origin of 

 the more marked types of the North American arborescent flora; and, 

 by correlation, to recognize the climatic circumstances governing the 

 flora of the Cretaceous Dakota group, as nearly identical, for the tem- 

 perature, at least, with those of the North United States at the present 

 time. 



The Dakota group is overlaid in the West and to the base of the 

 Eocky Mountains by more than two thousand feet of measure, marine 

 formations only, characterized as Cretaceous by an abundance of animal 

 remains. The upper member is composed of heavy beds of black shale, 

 with species of Inoceramus, Baculites, Ammonites, Belemnites, etc. To 

 this are superposed the lowest strata of the great Lignitic, a series of 

 layers of sandstone and of clay-shale, with remains of marine plants, 

 well-preserved Fucoids, and minute fragments of land-plants. This big 

 sandstone, as it has been generally called, is overlaid hy the productive 

 Lignitic measures, whose distribution in beds of coal, with underlying 

 clays and overlying shales generally holding in their composition a pro- 

 fusion of fossil remains of land-plants, with intermediate beds of sand- 

 stone, etc., is remarkably similar to that of the Carboniferous meas- 

 ures. The comparison of these coal formations of different periods 

 would be very interesting now; but the present sketch has to be limited 

 to the consideration of the essential characters of the American geolog- 

 ical floras onl3^ 



From a theoretical point of view, it would seem rational to suppose 

 that, in ascending higher in the series of the geological formations, and 

 in coming nearer to the present epoch, we should find a constantly and 

 gradually more distinct relation between the ancient floras and that of 

 our time, and that, therefore, the plants of the Lower Lignitic, though 

 intimately allied to those of the Cretaceous, should still bear a closer re- 

 lation to those of the present North American flora than do the Creta- 

 ceous leaves. This is, however, not the case. The Lower Lignitic flora 

 has not as yet a single species identical with any of the Cretaceous, and 

 even very few have a distinct relation to them. Its more essential char- 

 acter is marked by the presence of Palms, whose remains, especially 

 those species of Sabal, are in profusion, though appearing here as the 

 first representatives of this family, at least in the geological ages of this 

 continent. As seen by some of their trunks and leaves, they aris of 

 great size, and in such a proportion that at some localities, as at Golden 

 City, Colorado Territory, they seem to have composed one-fourth of the 

 vegetation of that time. They are, moreover, present in the whole ex- 

 tent of the Lower Lignitic; specimens of their leaves having been collected 

 from Placiere Mountain in New Mexico, to Fort Union on the Missouri 

 Eiver, or from 3(3° to 49° of latitude. In the Lower Lignitic, and in con- 

 ^ nectiou with the Palms, are leaves of Ficus, Cinnamotnum, Magnolia, 

 Myrica, Quercus, Platanus, Biospyros, 3£amnus, Viburnmn, etc., rather 

 related by their forms to southern than to northern types. The prepon- 



