June 20, 1878] 



NATURE 



191 



Miocene, and indicates a temperate climate such as that 

 now prevailing in the middle zone of the United States, 

 as from Ohio to North Alabama. The larger number of 

 its species are identified with, or analogous to, those of 

 Greenland, Spitzbergen, and Alaska, whilst a few arc related 

 to Pliocene plants, and three species are still living. Mr. 

 Lesquereux devotes the concluding pages of his ATork to 

 proving the Tertiary age of the Lig^itic series. If, as he 

 states, he has correctly ascertained that, in the first or 

 lowest group, 120 species represent Tertiary, and only six 

 can be considered at all as Cretaceous forms, he has 

 made good his case, and all European palaeontologists 

 will agree with his views as to the age of the Lignitic. 



The study of a very large series of British Eocene plants 

 in my own collection, from well-defined horizons, has 

 enabled me to draw somewhat different conclusions from 

 those of Lesquereux and Heer. Unfortunately no great 

 and undoubtedly Eocene flora has ever been described or 

 published, and I therefore use the Bournemouth as a typical 

 Eocene flora. The flora of Ceningen, made so familiar to 

 us by Heer, is a typical Miocene flora, and although most 

 unlike a true Eocene flora, contains many plants common 

 to other isolated fragments of strata which contain mixed 

 floras, that is, floras with percentages of Eocene as well as 

 Miocene plants. There being no typical series from the 

 Eocene available as a standard of comparison, the plants 

 common to the Miocene have alone been taken to deter- 

 mine the age of these beds, and the unknown Eocene 

 forms have thus been enrolled as Miocene, and in their 

 turn used to identify other still more distinctly Eocene 

 beds as Miocene ; much in the same way as the Barton 

 beds were formerly identified, from their possessing a 

 few species in common, as London clay, and the species 

 peculiar to the Barton horizon subsequently made use of 

 to identify Calcaire-Grossier and Bracklesham beds in 

 their turn with the London clay. The errors which have 

 thus possibly been committed even by Heer, who has 

 been led to class all the many floras he has so ably 

 described, either as Cretaceous or Ivliocene, were therefore 

 unavoidable, and scarcely reflect upon his judgment. 



The Lower Lignite is, in my opinion, undoubtedly 

 Eocene, and probably contemporaneous with our London 

 Clay or Lower Bagshot. The sudden incoming of palms 

 and European plants of tropical kinds, and of mammals, 

 and the displacement of the indigenous and temperate 

 flora and of the lingering Dinosavurians, is evidence clear 

 and unmistakable, that the continents became united at 

 this period. Simultaneously with this sudden increase of 

 temperature in America we find a corresponding increase 

 in Europe, as seen on comparing together the faunas of the 

 Thanet Sands and London Clay. The increase was in all 

 probability due to the final rise of a land barrier completely 

 shutting out all the cold northern currents, which at the pre- 

 sent day set towards the equator, and so materially modify 

 the ocean temperature. I think we are thus able to fix 

 the comparative age of the Lower Lignitic, which, being 

 upwards of 2,000 feet thick, probably required a great 

 part of our Middle and Upper Eocene period for its 

 deposition. I regard the second group as of our Upper 

 Eocene age, and the third and fourth groups as Miocene. 



Now comparing Dicotyledonous leaves of the Lignitic 

 flora with those of Arctic regions, we find several, eren 



of those from the Lower Lignite, common to both. The 

 greater part of these are included in the following list : — 



Viburnum Whytnperi, Heer. 



Fraxinus denticulata, Heer. 



Diospyros brachysepala, Al. Br. 



Andromeda Gray ana, H. 



Cissus tricuspidata, H. 



Vitis Olriki, H. 



Ficus iiliccfolia, Al. Br. 



We find common to the Lower Lignitic and the Mio- 

 cene of Switzerland — 



Querciis neriifoiia, Al. Br. 



„ chlorophylla, Ung. 



„ Godeti, H. 

 Salix angtista, Al. Br. 

 Populus melanaria, H. 



„ mtitabilis, H. 

 Ficus tilicBfolia, Al. Br. 

 Cinnatnonum Scheuchzeri, H. 



,, polymorphttm, Al. Br. 



Daphnogene anglica, H. 

 Diospyros brachysepala, Al. Br. 

 Cor mis Studeri, H. 

 Berchetnia multinervis, Al. Br. 

 Rhamntts alaiernoides, H. 

 „ rectinervis, H. 

 ,, Rossmdssleri, Ung. 



or far more than sufficient to have identified the forma- 

 tion with Miocene had its true position not been other- 

 wise ascertainable. None of these are, however, very 

 distinctive leaves, and, with very few exceptions, they 

 might, had the English Eocene Flora been published, 

 have been referred to it with greater approximation to 

 certainty. The exceptions are of little value, C. Scheuch- 

 zeri being identified on half a leaf, while the references 

 to P. muiabilis and C. polymorphum are extremely doubt- 

 ful. The truth is that so many of the ovate and lanceolate 

 leaves of the Miocene and Eocene resemble each other 

 that it would be easy to compile a sufficient list to refer, 

 with plausibility, any given flora to either age, according 

 to the author's fancy. 



I have not yet had leisure to enter more minutely 

 into the question, but it appears to me that the fact of 

 a proportion of the Lower Lignitic leaves, which are of 

 undoubted Eocene age, being also found in the Arctic 

 floras, and the untrustworthy nature of the evidence 

 on which these have been referred to Miocene, still 

 leaves the question of their true age, on palaeo-botanica 

 evidence, unsettled. We know that in Eocene times 

 these regions were land, and that floras existed upon 

 them, and passed from one continent to the other, whilst 

 in Miocene times, from the decrease in temperature, we 

 infer that the submergence of this bridge had com- 

 menced. Further, the high temperature in the Eocene 

 time would have permitted a temperate flora to grow in 

 these latitudes, and the Miocene temperature would not. 

 Lastly, it is difficult to conceive that the same quality o 

 flora could have grown contemporaneously in latitudes so 

 widely different as the United States and Greenland or 

 Spitzbergen, but there is no difficulty in realising that a 

 decreasing temperature, such as prevailed in the Miocene 

 would hare gradually driven the northern forms south- 

 ward, and thus the very similarity of the Miocene flora 

 of America to that of the Arctic circle renders it unlikely 

 that they were of the same age. J. S. Gardner 



