6 BRITISH EOCENE ELORA. 



in which the plants grew is rarely taken into account. It appears, for instance, 

 that the floras of our Middle Eocene enjoyed an exceptionally favorable climate, moist 

 and in close proximity to a very warm sea. Those of Aix, though so much farther 

 south, had a dry and less favorable station, and therefore contrast with ours in point 

 of luxuriance, for there moisture-loving plants seem to have been excluded until a later 

 period. Other stations, such as Bovey-Tracey, eighty miles from Bournemouth, situated 

 among hills which probably helped to furnish the materials of the Bournemouth deposits, 

 may have stood considerably above the sea-level, and would present in consequence a 

 slightly more temperate flora, which has been interpreted to be younger. In comparing 

 American plants it should be remembered that the climate of the United States, even 

 during the Eocene, was, latitude for latitude, cooler than that of Europe. Other examples 

 might be instanced, but sufficient is stated to show that so far from it being possible to 

 define the relative ages of fossil floras by their mere comparison, this class of palaeonto- 

 logical evidence requires perhaps exceptionally philosophic treatment. A knowledge of 

 the probable conditions under which Tertiary plants lived is an essential preliminary to 

 their study, and a few considerations under this head will be of use. 



The curves of the lines indicating the limits of trees towards the Pole, as well as of 

 the isotherms, show very strikingly the cooling effects of descending Arctic currents upon 

 the surrounding land. Wherever Arctic water penetrates in gulfs or bays, or finds 

 egress, the limit of trees is deflected ten degrees south ; while currents flowing northward 

 carry the lines with them by fending ice off" the shore. We need not seek farther than 

 Iceland for an illustration of the changes in climate that a temporary diversion of an 

 Arctic current will produce. In 1757 Greenland ice remained on the north shores of 

 Iceland, and produced throughout the summer a cold equal to that of the depth of 

 winter, which did not diminish under a mid-day sun, the air remained misty and snow 

 fell frequently throughout May and part of June, preventing the growth of grass. The 

 previous year snow fell to a depth of two feet on the 26th June, and continued to fall 

 frequently during July and August with excessive cold. The ice left on the 2Gtli 

 August, and the snow then melted. A similar visitation in 1753 had destroyed nearly 

 the whole of the cattle, and the ponies ate wood, peat, and flesh. 



It seems from this example, and from the difference between the isotherms of 

 Europe and America in the same latitudes, that ice-laden currents are of themselves 

 capable of lowering temperatures by twenty or thirty degrees. It is a question whether 

 the exclusion of ice altogether from the North Atlantic might not be sufficient to raise its 

 temperature generally by some 20° Fahr. It is admitted that continuous land did exist 

 during the Eocene period between Europe and America, in about latitude 70°, and this 

 land must have completely excluded floating ice from seas to the South of it. If this 

 cause were admitted to be capable of producing so great an effect, no other explanation 

 of the presence of the floras that are met with in Greenland and in England would be 

 required. The sudden rise seen in the temperature of the London-Clay period, both in 



