AGE OF THE LIGNITIC DETERMINED BY ITS FLORA. 351 



same for very long periods. But lluil, a coiiiparativi'ly large iiuml)er of 

 land or fresh-water plants, subject to uiodificatious or forced to migrations 

 by atmospheric changes, may be preserved identical through the lapse of 

 time indicated by the thickness of the measures heaped along Bitter Creek, 

 has not been proved by as positive an evidence as we have it here. It is 

 scarcely possible to hazard a conjecture upon the length of time indicated 

 by the building-up of these intermediate measures. Evidently of a sliore 

 formation, the accumulation of their materials may have been more rapid 

 than tor the deposits at the wide bottom of the sea. The strata, however, 

 in their successions, are not merely sandstone beds of great thickness, some- 

 times blackened by small fragments of land plants ground by the waves, and 

 mixed with other materials, but beds of clay built np of swampy deposits 

 of long duration, and especially lignite or coal-beds, still more clearly denoting 

 the slow progress of the work. 



The relation of the floras of Black Buttes and Point of Rocks, in spite 

 of the long ])eriods of time which separate them, as proved by the series of 

 intermediate formations, is, therefore, a liict of great interest to botanists, and 

 not less so to geologists; for it bears upon the perplexed question of synchron- 

 ism, and at the same time is an important point of comparison in regard to the 

 geographical distribution of our present flora. But here it has to be consid- 

 ered merely in connection with the determination of the age of the Lower 

 Lignitic. The Cretaceous Dakota group is separated from Point of Rocks by 

 a thickness of strata about the same as that which is marked between Point 

 of Rocks and Black Buttes. Nevertheless, between the floras of the Nebraska 

 and Kansas Cretaceous and those of Point of Rocks and Black Buttes, we find 

 few analogous types and not a single identical form. The erosions may have 

 indeed considerably thinned the marine .strata representing the Cretaceous 

 above the Dakota group, but that cannot lessen the strength of the deductions 

 made from the total disconnection of the two floras, one of which denotes, by 

 its essential characters, a marked dissimilarity of atmospheric circumstances, 

 a weighty evidence, if not a positive proof, of a change of epoch, not in the 

 sea perhaps, but at least upon the land. 



It is useless to repeat that as yet no i-emains of deep marine Cretaceous 

 types have been discovered in thewhole Lignitic measures abovePointof Rocks. 

 We may admit, however, tliat wliile the Tertiary age was at its beginning rep- 

 resented as a land formation.as seen by its flora, a Cretaceous marine fauna may 



