246 



as yet by about eighty species, has only ten species common to the three 

 former groups ; and even six of these are so-called omnipresent species, 

 or present in the whole thickness of the Tertiary. It has, however, 

 thirty-two of its species identical with species of the European Upper 

 Miocene, and none positively identical with any living at our time. It 

 cannot, therefore, be referred to the Pliocene age. 



This review should not be closed without a few remarks on the floras 

 (too little known as yet) of some more recent geological epochs. A large 

 number of specimens, representing about forty species, have been 

 obtained, in a very fine state of preservation, from the chalk-bluffs of 

 Nevada County, California, referred to a Pliocene formation. These 

 plants are related in a more evident degree to those of the present flora 

 by their general facies and by a few identical species. Except two or 

 three of the types referable to Asiatic (Japanese) origin, they are Ameri- 

 can, especially related to species of the eastern slope of the continent, 

 and, by a few, to species still in the flora of the Eockj^ Mountains. 

 Remarkably enough, some genera, like JJlmus, for example, are repre- 

 sented in the specimens of the chalk-bluffs by a large percentage of 

 the remains, while they are at our epoch absent from the flora of Cali- 

 fornia. A formation of apparently the same age is present in the chalk- 

 bluffs or clay-beds bordering the Mississippi Eiver below the mouth of 

 the Ohio, near Columbus, Ky. Too few species have been as yet 

 collected from that formation, and thus nothing positive can be said 

 about its flora. Its relation seems to be very marked with the present 

 flora of the Southern States, or rather of the Gulf shores. Flanera 

 cymelini^ Quercus virens, and species of TJlmus have been described from 

 that locality, i^o doubt that when its fossil plants have been collected 

 very valuable indications will be obtained from their study in regard to 

 their relation with more ancient types and transitional forms from the 

 old ones to those of the present flora of North America. 



To this chain other links can be added in the future. The Drift of 

 the West, in Ohio and Indiana especially, is interstratitied by deposits of 

 leaves and trunks ; also, by peat formations, where a profusion of vege- 

 table remains are obtainable for studying the progress of the vegeta- 

 tion during the Glacial epoch. Still nearer the present time, and corre- 

 sponding to the Terrace epoch, thick beds of leaves heaped along the 

 Lower Ohio Eiver are open to the researches of the paleontologist. There 

 the leaves, still undecomposed, mixed with sand and clay, are pressed 

 together in banks, which are cut in stages like terraces. They represent 

 mostly species of our flora; but some differences of character may be 

 found there as a clew to the mode and progress of modification under 

 various and appreciable kinds of influences. 



This would complete the chain of evidence in regard to the develop- 

 ment and succession of the types of the North American flora from the 

 Cretaceous to the present time. What an admirable record is in reserve 

 for the botanist who shall be disposed to give his time to the noble task 

 of deciphering and of transcribing it ! There is certainly not a country 

 in the world where the study of the geological floras, of their characters 

 and successions, can be pursued with more advantage, and furnish at 

 the same time more important and more trustworthy documents, than 

 in the United States : for, except the Jurassic and the Permian, the 

 groups of the geological floras are represented in our formations by 

 abundant and generally well-preserved materials, and the distribution 

 of the strata is so distinctly marked that their age and succession are 

 easily determined. Therefore, the deductions which are likely to be 



