CBETACEOUvS FLORA. 7 



Cretaceous floras Isolated.] 



are identified in the Cretaceous of Germany, none in the Dakota group, which has 

 species of oaks of a different type. Of the Laurinece, Greenland has seven ; we have 

 seventeen in the Dakota group ; Europe has none, while species of Credneria are 

 numerous in the Quader sandstone of Germany, and one only has been described 

 from Greenland and one from the Dakota group. The same differences are observed 

 in some of the other groups, while some present a remarkable degree of affinity. 

 The same remark is applicable to the distribution of the plants upon the land 

 surface of the formation in North America. Kansas, for example, has many species 

 which have not been found in Nebraska or in Colorado, and vice versa. In the small 

 number of species described here below from Minnesota, there are eight which have 

 not been found elsewhere and ai-e considered as new. The geographical distribution, 

 as far as it is known at the present time, is really more complex and varied than it 

 is in the vegetation of the present epoch. The vegetable remains are not found 

 strewn over large surfaces of the land, as if they were derived from forests of wide 

 extent, but over small isolated areas, more or less distant from each other, as if the 

 leaves found there had fallen from groups of trees growing separate upon small 

 islands or around wood swamps of small extent. And generally the plants of each 

 area are of the same or of related species or represent only few species or genera, 

 each locality having some plants proper to it. At one place the Sassafras abounds ; 

 at another the Laurinete; still at another the Liriodendron, or species of another 

 genus or family. Such a distribution does not agree with what it should be for 

 plants derived by evolution of one or more species, as the plants of the same kind or 

 varieties should, of course, remain together or follow the same range and direction 

 in their distribution. It is worth remarking that as far as it can be observed neither 

 the geological features nor the conditions of the atmosphere of the Middle Cretaceous 

 have been subjected to great changes. The cataclysms caused by volcanic agency, 

 and the gradual elevation of the chain of the Rocky mountains, have come after the 

 Cretaceous. Nothing in the vegetation of that epoch indicates great and prolonged 

 disturbances of the atmosphere. In the lower series of the vegetable scale, the ferns, 

 the conifers and the Cycadecp are of the same type and some of the same species, as 

 in the Wealden, the Vernsdorf shale of the Lower Cretaceous, the schists of Korne, 

 and those of the Cenomanian of Atane in Greenland. And from the time when 

 the dicotyledonous plants appeai'ed, or when we tind them predominant, some 

 of the types which may be called primitive, as it is not possible to refer them by 

 derivation to some anterior ones, have continued in the different groups of floras 

 through the Upper Cretaceous and the Tertiary to the present epoch, modified, indeed, 

 but distinct enough .to be recognized in many genera and species of the living flora 

 of this country. 



