422 GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



some species and representatives of genera which have been described 

 irom the Dakota Cretaceous, Cinnamomum Scheuchzeri, Liriodendron, and 

 Sassafras, and it has also some of its species identical with those of the 

 Arctic Miocene. Its fades, therefore, positively indicates a somewhat 

 colder temperature than that of Lower Lignitic, 



With the third group, the palms have disappeared entirely, as well as 

 the subtropical types of the Lower Eocene. Its flora has also a Liquid- 

 amljar, L. gracile, closely allied to a Cretaceous species, L. integrifoUum, 

 and with this it has a more marked predominance of arctic forms or 

 species identical with those of the Miocene of Greenland and Alaska, as 

 seen in the remarks on the tables. The lowering of the temperature is 

 there still more marked than with the second group. In the whole ex- 

 tent of the Lignitic formations we can see, therefore, from the character 

 of the successive flqras, a slow decrease in the degrees of tempera- 

 ture, accountable, it seems, to the diminution of atmospheric humidity 

 in proportion to a gradual consolidation and drainage of the laud. The 

 same phenomenon is indicated by the deposits of Lignitic beds, which, 

 though of as great thickness iu the second and third groups, cover less 

 extensive areas. 



The fades of the flora of the fourth group evidently represents the 

 colder climate of a mountainous region, by the superabundance of 

 conifers as the essential constituents of the forests of that epoch. I^ 

 has, besides, many species of shrubs, Salix, Myrica, Gomptonia, Ilex, Elms, 

 which generally form the undergrowth of pine-woods, or border the 

 swamps and streams intersecting them, and in accordance, a less pro- 

 portion of trees with deciduous leaves. This vegetation of the Uj)per 

 Tertiary recalls by its character that of the Adiroudacks of Xew York or 

 of the Black Mountains of North and South Carolina, where each knob 

 is overgrown by one species of conifers, here and there intermixed with 

 poplars, birches, sometimes oaks and beeches, and where the under- 

 growth scarcely allows to penetrate in the dark recesses of the forests. 

 xls remarked iu describing and comparing the species of this group, the 

 flora of each place where fossil plants have been obtained is composed 

 of some species of conifers which are not represented at the other local- 

 ities. In the forests of the plains, conifers of a same kind are generally 

 extensively and uniformly distributed, covering wide areas, as in some 

 l^arts of Europe and of North America, especially in the maritime pine- 

 V, oods of the South and the northern forests of the cold plains of Canada, 

 Norway, &c. But in the mountains even at our time the forests are 

 composed of numerous groups of a predominant species of conifers, 

 represented in separate and limited areas, and varying in accordance to 

 altitude, exposition, degree of declivity, &c. The limitation of conifer- 

 species to diifereut localities of our Upper Tertiary is thus characteristic 

 of a mountain-flora. 



In admitting, as positively proved, the exact and constant relation of 

 the flora of a country or of a land surface with the climatic circum- 

 stances of the same localities, it iS easily understood how doubtful are 

 the conclusions taken concerning the relation of geological epochs in 

 comparing the fossil floras of two continents. -The four groups of our 

 Tertiary are characterized by a succession of types bearing constant 

 increasing analogy to those our present flora without the admixture of 

 foreign vegetable forms, which imprint some local floras of Europe with 

 peculiar and distinct /acies. This indicates for this continent a long con- 

 tinuance of the same climatic circumstances without notable modifi- 

 cation. These circumstances have not been of necessity the same in ■ 

 Europe during the same period of time. There may have been, for ex- 



