1849.] AUSTEN ON THE VALLEY OF THE ENGLISH CHANNEL, 95 



English Channel suggests, I have endeavoured to confine myself to 

 those which belong to pure geology. I might however have borrowed 

 great aid from a branch of inquiry first imagined by Prof. E. Forbes 

 — that of the relation which an existing flora may bear to past geo- 

 logical changes * . I would suggest whether it may not be sufficient 

 to reduce the existing fauna and flora of this country to two periods 

 of origin ; one which has come in since the period of the glacial drift, 

 the other that which forms the local character of several districts, 

 which we have shown were insulated in the pleistocene ocean, and 

 which flora has outlived all subsequent changes. In this way the 

 characteristic plants of the south of England and Ireland f will be the 

 residual portion of that of the pliocene period, which corresponds 

 with that of the greatest amount of area and elevation. The Scandi- 

 navian character of the floras of part of Wales, the Lake district :|:, and 

 more particularly the north of Scotland, will, under this supposition, 

 be the remains of the alpine regions of the same period. 



The condition of surface prior to the overlap of marine pleistocene 

 accumulations is indicated by the remains of vegetation so constantly 

 found beneath the drift with elephants' remains ; but traces of this 

 vegetation are also met with (as over the Wealden) in tracts which 

 the pleistocene waters never reached. Of the remains of this ante- 

 cedent vegetation, the Pinus sylvestris is perhaps the most charac- 

 teristic. Undoubted trees of this species, and of great size, occur 

 over the surface of the Wealden denudation, buried in old peat bogs, 

 and are remarkable for the great thickness of their bark — a character 

 which becomes marked in proportion as the tree advances to a colder 

 region. 



The period of the terrestrial conditions of greatest cold over the 

 area of Great Britain would therefore be, when it was part of an area 

 of much greater extent, and at a much greater elevation. With this 

 extended area, and absence of internal seas, there would be, as com- 

 pared both with the pleistocene and present condition, an excessive 

 or continental climate. The character of the flora of such a geo- 

 graphical condition would be at the same time more southern and 

 more northern. Much of what now constitutes upland sandy tracts 

 devoid of vegetation, from insufficient moisture, would then have been 

 included in the regions of pines and forests, of which the buried 

 Scotch and spruce firs are the remains. 



In like manner the period of the marine conditions, with a meagre 

 fauna, would be that of the greatest amount of depression and ex- 

 panse of sea. From what has gone before we may infer that the 

 process of submergence during the pleistocene period was gradual 

 and progressive from north to south, the marine fauna being such as 

 waters coming in such a direction would bring with them ; the ter- 

 restrial and marine periods of minimum temperature do not therefore 

 correspond in geological time. 



Even at the period of greatest depression, the Pinus sylvestris 

 might continue to hve on over the northern insulated part of the 



* Memoirs of the Geol. Survey, vol. i. f Floras i. ii. and iii. 



t Flora iv. 



