CONTAINING FLINT IMPLEMENTS, AND ON THE LOESS. 
289 
at a certain time towards the end of the glacial period, a temperature of 12-|° below that 
of the present day had supervened, the further extension of the glaciers may have been 
thereby checked; and as the present mean annual temperature of the S.E. of England 
and N.W. of France may be taken at 50°, this would have made it equal to 37^° — the 
mean annual temperature of the two stations before named, Moscow and Quebec, being 
respectively 40 o, 02 and 41°-85*, and that of Cumberland House (Northern America) 
being 80°. This mean annual temperature would agree with the conclusions at which 
we had arrived with respect to a mean winter temperature below 20° and above 5°, or 
possibly between 10° and 15°, having prevailed during our high-level gravel period. 
If there had been no amelioration in the climate at the period of the high-level 
gravels, the permanent ice and snow accumulated during the preceding long-continued 
and severe cold on the hills and mountain-chains of Europe would have remained with- 
out change, and the discharge of the rivers would only have been in proportion to the 
annual rainfall, whatever that was; and if that fall were not excessive, we should 
have no extraordinary agents in operation beyond the winter frosts and snow and the 
attendant spring floods. But if we suppose that (as must necessarily have happened at 
some time between the glacial period and the recent period), owing to a further improve- 
ment in the climate, the winter temperature became permanently and most probably 
gradually higher, then it would follow that during each recurring spring the rivers 
would have had their former ordinary discharge increased by the addition to the 
annual rainfall of a certain proportion of the snow and ice stored up during the 
former cold period. This quantity might have been equal to the accumulation of one, 
two, or more winters, according as the rate of elevation of the mean annual temperature 
was slow or rapid. In all valleys connected with mountain- chains the result of these 
climatal changes must have greatly increased the power of the annual floods — whence 
the greater excavation of the valleys connected with such regions. In our case, however, 
the extreme conditions do not generally apply, though I believe the foregoing general 
cause influenced the results. Most of the valleys we have to investigate are not 
connected with areas of old glaciers. The Waveney, Ouse, and Somme are not so 
connected. No traces of old glaciers are recorded even in the Ardennes, and those of 
the Morvan have been contestedf, although I think without sufficient reason. Never- 
theless with the degree of cold we suppose to have existed at this subglacial time, the 
mere winter accumulation of ice and snow on the higher ranges of hills must have 
been large. The effects of the greater water-power observable in the valley of the Oise, 
and of the Seine more especially, may be due not only to the height of the ranges of 
hills in which they take their source, but also to the larger areas of drainage. 
Starting from the point that the high-level gravels of our district are of an age sub- 
sequent to the maximum period of cold, that they mark a period during which the 
winter temperature was gradually becoming less rigorous, and that the excavation of 
* The various mean temperatures are from Dove’s valuable Tables, Eeports of Brit. Assoc, for 1847. 
t Bull, de la Soc. Geol. 2nd series, vol. ii. p. 683 (1845). 
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