I 



Mr. J. Prestwich on the Drift Deposits. 67 



nowhere so abundant in the low- as they are in the high-level gravels. 

 The pointed lance-shaped form with blunt butts of the latter is 

 almost wanting in the former, whereas the ovoid disks of Menche- 

 court are rare at St. Acheul ; again, flakes or flint-knives are com- 

 mon in the low-level gravels and rare in the higher beds. Of the 

 twenty-four specimens found in the low-level gravel at Paris, twenty- 

 two are mere flint-flakes. The author is disposed to attach some 

 value and significance to this difl"erence of form, and observes that, 

 admitting the climate to have become less severe during the low- 

 level gravel period, it would follow that the necessity of having the 

 strong ice-chisels would have diminished. In all these cases we are of 

 course much limited to conjectures, seeking to make them in accord- 

 ance with what we know of life under like conditions, and guided by 

 the probabiHties of the concurring circumstances. The mode of 

 distribution of the flint-implements at the two periods certainly 

 seems to aiford some grounds for believing that the difference of form 

 may arise from difference in the pursuits and occupations of the pri- 

 mitive tribes by whom they were used — pursuits necessarily and 

 primarily influenced by the climate and life of the period. 



Concluding Remarks. — The question of time is then entered upon, 

 and it is shown that the flint-implements must be carried back 

 through the periods of the low- and of the high-level gravels, and 

 that they must be considered to be antecedent to the excavation of 

 many of our great river-valleys. All these phenomena indicate 

 periods of long and great changes. The author only slightly touches 

 upon the formation of the loess, which he concludes to be the result 

 of temporary floods ; and he remarks that, so far as the question of 

 the antiquity of the fluviatile gravels is concerned, little value need 

 be attached to the additional element presented by this covering of 

 loam and brick-earth. This deposit is succeeded by the alluvial 

 beds of the valleys connected more immediately with our own times. 

 With regard to a measure of time, the author does not consider that 

 either the excavation of the valleys or the life evidence of the periods 

 furnish available data; nor does he admit the formation of the 

 channel between England and France in the calculation ; and he gives 

 reasons to show that this channel is of older date than generally as- 

 sumed, and that the separation existed at the time of the high-level 

 gravels, and had attained somewhat of its present dimensions at the 

 time of the newer gravels. Most of the land and freshwater shells 

 and the Mammalia had crossed over at a period anterior to this ; 

 and, as even now at the Island of Saghaleen in lat. 52° N., the 

 narrow strait freezing during the winter would admit of the passage 

 of large land animals and man during the cold periods following the 

 more extreme glacial conditions. 



The author, however, suggests two new modes by which he con- 

 ceives that eventually some approximate and more exact estimate 

 may be made both of the age of the high-level gravels and of the 

 lapse of time since the extreme glacial period, and embracing there- 

 fore the several periods under consideration. At present the evidence 

 is only sufficient to indicate the possibihtics of the problem, but it 



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