LIKNEAN SOCIETY OF LONDON. 85 



plants of the Cromer beds : — Isoetes lacnsfris, Linn,, represeuting 

 a plant of a colder region ; and the still moi-e prouounced Betitia 

 nana, Linn., now confined in Britain to mountains in Scotland. 



With the exception of these two plants, the Cromer Forest- 

 bed, which contains as I have said the earliest records of our 

 existing Flora, presents us with a fair selection of that great 

 section of our present vegetation which we refer to the Grermanic 

 or Central Europe type. This temperate Flora, however, dis- 

 appeared befoi'e the advancing ice-cap and its accompanying 

 arctic temperature. In the presence at Cromer of Sali.v polaris, 

 "Wahlenb., now found only within the Arctic Circle, and Hypnum 

 turc/esoens, an Arctic moss, we have the evidence of this extreme 

 cold. These two plants were discovered by Prof. Nathorst, of 

 Stockholm, who concluded that some evidence of the change from 

 the temperate climate indicated by the plants of the Forest-bed 

 to the severe cold of the superimposed Boulder Clay might be 

 found. And he was successful, in a visit to Cromer in 1872, in 

 discovering the Arctic Salix and Hypnum in a bed immediately 

 below the Boulder Clay. 



But the cold was too severe even for the Arctic plants, and 

 they were puslied out before the still advancing ice-cap. There 

 then reigned a death over the land as complete as on Krakatao 

 when six yeai's ago it was covered with the thick layer of hot 

 pumice. It is difficult to realize the condition of the North of 

 Europe at this time. INot only was the surface of the land 

 covered with ice, and great glaciers ploughed their way down all 

 our mountain- sides, but tlie mountains of Scandinavia gave 

 origin to huge fields of land-ice which spread over the North of 

 Germany and, crossing the North Sea, pushed their way into our 

 own land until they were arrested by the laud-ice of Britain. 

 They have left as witnesses of their presence numerous fragments 

 of the granitic rocks of Sweden enclosed in the Boulder Clay of 

 the South-east of England. 



The physical conditions which drove out of the land the Arctic 

 plants would not permit the continuance of the Spanish plants 

 which were supposed to have reached Ireland, nor the French 

 plants which had been established in the South of England. The 

 creation oi preylacial laud-bridges across the Bay of Biscay and 

 the English Channel could not have brought the present repre- 

 seutatives of the French and Peninsular floras to the South-west 

 of Ireland and the South of England. 



The Ice Age in Britain was not a continuous period of un- 

 changing low temperature. There is physical evidence that in 

 its covirse there was a very decided advance in the temperature 

 and a coi'responding retreat of the ice-covering. The interval of 

 improved temperature lasted long enough to permit the exposed 

 surfaces of the laud to be occupied by vegetation. The plants 

 followed the retreating ice. They have left their remains in beds 

 intercalated with the true Boulder Clay in the valleys of the Forth 



