LssQUEEEux.] PALEONTOLOGY LIGNITIC FLOEA SPECIES. 423 



ample, at the epoch of the Lower Tertiary or Eocene, a higher tempera- 

 ture, iuiiuenced by proximity to the sea, by its currents, by slanting 

 areas exposed to the snn, &c., and of course a corresponding flora, 

 Indo- Australian or tropical, &c., while, under different influence, we had 

 at the same epoch a more moderate temperature and a flora with homol- 

 ogous types, related to those appearing later in Europe, when the tem- 

 perature was at a lower degree, in the upper Miocene epoch, for example. 

 This explains, of course, the non-correlation of vegetable types at 

 epochs which are recognized as synchronous by their animal fossils, and, 

 therefore, contract our deductions of synchronism of strata, as indicated 

 by identification of fossil remains, into more narrow limits. It is prob- 

 ably for this reason that, in comparing the data furnished by our ancient 

 floras with those of Europe, we have constantly recognized a kind of 

 precedence of types which may be merely the expression or exposition 

 of a difference of climatic circumstances at the same epochs. Of this, 

 however, we have to learn a great deal more on those floras of old 

 before we are able .to take any reliable conclusions, and for this reason, 

 also, it is of importance to limit our deductions on what we may learn 

 in considering our North American fossil floras. 



Paleontological data, animal and vegetable, have demonstrated, for 

 the geological times, as far up as the Lower Tertiary, or at least the Upper 

 Cretaceous, a uniformity of climate over the whole north hemisphere, 

 from the pole to the equator, if not over the whole world. Tbe causes of 

 this phenomenon are multiple and not yet satisfactorily explained. In 

 the flora of the Dakota group, and also in that of the second and third 

 groups of the Tertiary, this isothermal fades is remarkably proved 

 by identity of genera with those of the flora of tbe north, or as far 

 up as remains of fossil plants have been found, especially with that of 

 Greenland. The Cretaceous flora of Come, described by Heer in his 

 Arctic Flora, is represented only by species of ferns and conifers, which 

 do not have any relation to the plants of tbe Dakota group, except per- 

 haps by one single species, Sphenopteris JoJmstrupi, which is compara- 

 ble to Hymenophyllnm cretaceuon. This flora of Come may be refer- 

 able to a lower stage of the Cretaceous, as it has no remains of 

 dicotyledonous leaves. In an upper flora of the same country. Profes- 

 sor Heer finds mostly dicotyledonous leaves, and recognizes them as 

 referable to many of the genera represented in our Dakota group. As 

 the memoir of those plants is not yet published, it is not known how 

 intimate the relation may be ; but the generic identity is enough already 

 to indicate analogous climatic circumstances in Greenland and North 

 America at tbis Upper Cretaceous epoch. The flora of our Lower Lig- 

 nitic, the oldest of the American Tertiary, is as yet without rela- 

 tion with any northern flora known until now. But that of the second 

 group and of the third are related, as remarked above, with the Miocene 

 Greenland flora by a number of species and typical forms, which are 

 characteristic enough to show that a same climate influenced at this 

 epoch the vegetation of both countries. Therefore, from this, it seems 

 that as far up as the Miocene period the isothermal zone extended from 

 the tropic to the pole, or that at that epoch the same climatic circum- 

 stances have governed the vegetation of the North American continent. 



The relation of the floras to the climate being forcibly recognized in 

 local differences, or analogies of vegetable forms, it suggests another 

 question, that of the origin of the groups of vegetables characterizing 

 either different stages of the Tertiary or different localities of the sauie 

 epoch.* 



Our flora of the Dakota group has for its essential representatives 



