234 VEGETABLE COLONIZATION. 



from England. They still exist in the mountains of Cumberland, 

 Wales, and, above all, of Scotland, where they find a climate analo- 

 gous to that of their native regions. 



At the end of the glacier period the British isles began to rise 

 slowly from the bosom of the waves. Everywhere we observe on their 

 acclivities, terraces or lines of ancient coast indicative of intervals of 

 repose which have interrupted this gradual elevation. To comprehend 

 properly this phenomenon, it is necessary to figure to one's self, not a 

 simple upheaval of the shores, the submarine tracts remaining im- 

 moveable, but a simultaneous movement of both, rising proportionally 

 above their ancient level. This upheaval it is which has modelled 

 the existing outline of the British isles and determined the configura- 

 tion and the depth of the surrounding seas. The depressions have 

 become less profound and the higher grounds have emerged. Hence 

 a change in the maritime fauna. The sea being warmer, its shores 

 have been occupied by the species which now people them. But the 

 change of temperature being much less sensible at great depths, the 

 animals of the glacier period have been able to subsist in that situa- 

 tion. Thus, says Mr. Forbes, in depths where the sounding-line 

 announces from 525 to 656 feet, the dredge brings up the molluscs of 

 the Arctic seas, and even a large number of shells which exist in a 

 fossil state in the drift or deposit of the glacier period which covers 

 the northern portion of the British isles. From this assemblage of 

 facts Mr. Forbes concluded that the profounder seas of Britain conceal 

 populations which have existed there since the glacier era, in like 

 manner with the plants which crown the summits of the Scottish 

 Alps. 



During the whole of those two geological eras which we have just 

 been considering England, was united with France. The British 

 channel and straits of Dover did not exist. It is an admitted fact in 

 science, concerning which all geologists are agreed, that the separation 

 of England from the continent is an event comparatively modern and 

 perhaps cotemporaneous with the human race. Messrs. Constant 

 Prevost and d'Archiac have perfectly demonstrated it; the former by 

 pointing out the correspondence of the strata of chalk in both banks 

 of the channel, the second by demonstrating the identity of the con- 

 geries of rolled pebbles which overlies the chalk. This latter mass, 

 similar to that of our present rivers and streams, forms the most 

 superficial of the deposits, which was consequently formed after all the 

 rest. And as it is the same on both sides of the channel this layer 

 must have been deposited by the same current when the two countries 

 were still united. The separation took place later, occasioned, it 

 would seem, by the removal of the beds of chalk, which on either side 

 dip landwards and have an elevation towards the sea. 



At the dawn of the present era England, then, formed a peninsula 

 similar to that of Denmark, The climate and the surface of the 

 country were what they now are, and the plants of France and Ger- 

 many soon occupied the lately emerged lands. The hardy growths 

 of northern Europe possessed themselves of the greater part. Forests 

 as dense as those of Germany then covered the hills of England. 

 Stagnant waters collected in hollow places, and the mosses and fens 



