6 THE PALEONTOLOGY OF MINNESOTA. 



^Correlation of floras. 



In considering the question of the correlation of the plants recognized in the 

 succession of the floras since their origin, one forcibly arrives at the same conclusion, 

 the impossibility of explaining by antecedents the characters ot the vegetation of 

 the Middle Cretaceous, or rather the presence of the dicotyledons as its essential 

 element. From the beginning and in closely following the march of the vegetation, 

 we find an evidentdegree of parentage between the groups which disappear and those 

 which follow them. Thus the affiliation of the ferns of the Devonian to those of the 

 Catskill group ; then to those of the Subcarboniferous, is easily followed up into the 

 Permian, and still, by gradual modification through the ages, to the present epoch. 

 The great Lycopodiacece, Sigillaria, Lepidodendron, etc., gradually take a more marked 

 place in the vegetation of the palaeozoic times, have the highest degree of predomi- 

 nance in the Carboniferous, have their time of decline in the Permian, but continue 

 to be represented up to the present epoch by plants of the same kind but of small 

 size. The conifers also, which distinctly appear in the Permian by peculiar forms, 

 gradually becoming more predominant, constitute the essential vegetation of the 

 Jurassic, still remain in the present flora under somewhat modified forms. The 

 Cycadecp, apparently as old as the Lycopodiace(v, follow the same march of development 

 traversing the Carboniferous as an essential constituent of the vegetation, declining 

 in the Permian and by modification of some of their characters passing to the Cycadew 

 which then follow the same march as the conifers. There is, indeed, between the 

 Cordaitece and the Cycadece a marked difference, but the mode of gradual transform- 

 ation between plants which have such great analogy of characters may be easily 

 conceived. After following the gradual variations of types through the palaeozoic 

 time one may follow them still from the Cretaceous and see them also continued 

 upon the dicotyledons from their appearance through the cenozoic ages, to the pres- 

 ent epoch. Hence all the groups of vegetables appear from their origin as linked 

 together by a kind of putative affiliation ; but nothing like that can be seen to fore- 

 tell the appearance of the dicotyledons in the Middle Cretaceous ; the parentage is 

 derived from them in the hereafter, but none can be found in the past. 



As we see it in the Dakota group, in the formation of Atane in Greenland, and 

 also in the Middle Cretaceous of Europe, the Cenomaniari flora appears in its distri- 

 bution and in the general character of its groups, as a complex assemblage cf vege- 

 table types developed under the acting forces of a long series of ages. Though 

 a number of species are found identical at the different localities, the groups are 

 generally different in characters. For example, the four species of Populus of 

 Greenland are of the section of the coriaceous poplars, and of these, none is found in 

 the Dakota group. Of the seven species of Quercus described from Greenland, two 



