PROCEEDINGS OF GEOLOGICAL SOCIETIES. 



193 



the more tranquil accumulation of the middle sand-beds of St. Acheul, 

 and for the more numerous shells living there undisturbed. During flood, 

 the river rose probably to a height of twenty to thirty feet above its ordi- 

 nary level, as shown by the brick-earth (without gravel) deposited higher 

 up the hill on the road to Cagny. The next stage showed — 



2. The gravel beds of St. Acheul after they were left dry, except during 

 floods. — Here the valley had been excavated to several feet below the level 

 of the St. Acheul beds, but during floods the river still extended over 

 them and deposited the brick-earth. Ice-floes grounded and indented the 

 upper beds of sand and gravel, causing contortion of the strata. 



3. The river at the time of the formation of the low-level gravel beds of 

 St. Roch and Amiens. — The valley had now deepened to the extent of fifty 

 to sixty feet below the level of St. Acheul, and the low-level gravels of 

 St. Hoch were deposited under similar conditions, only that ice-action is 

 not so strongly marked. It is in these beds that the remains of the hip- 

 popotamus first appear. The flint implements found in them are of a 

 somewhat different type to those*of the higher level gravels. The flake 

 form is more prevalent. 



4. The low-level gravel of St. Roch, left dry except during floods. — 

 Here we have a repetition of the same state of things as found at St. 

 Acheul ; the shingle being covered up during floods by brick-earth or 

 loess. 



5. The valley at the present period. — The progress of excavation shows 

 the valley deepened to its full extent. The river has lost its old power, 

 its flood waters now rising only two to three feet, and its channel being 

 restricted within a very narrow compass. The old and rough channel left 

 at the end of the Quaternary period is covered to the depth of ten to 

 twenty feet by fine alluvial soil and peat. All the great pachyderms have 

 become extinct, but the reindeer, bison, and great fossil ox survived 

 during part of the more modern time. Almost all the small and fragile 

 land and freshwater shells have continued in uninterruped descent to the 

 present day. This fact seems almost conclusive against any general cata- 

 clysm having passed over the surface. This concordance between the 

 physical features and the contemporary life, and the capability the hypo- 

 thesis here offered gives of explaining each and every one of the pheno- 

 mena, affords strong presumptive proof of its truth. 



Before concluding, Mr. Prestwich observed that he might be expected 

 to say a few words respecting the age of the flint implements. Two ques- 

 tions were involved in this,— one the length of time elapsed since the 

 close of the period of the extinct mammalia ; the other, how far back into 

 that period the flint implements can be traced. In the description of the 

 sections, it had been previously pointed out, that the Quaternary period 

 could probably be brought down immediately to the time when our valleys 

 began their modern accumulation of silt and peat. The period of time, 

 therefore, first to be measured is that which has been required for the for- 

 mation of these latter deposits. On this point there is considerable dif- 

 ference of opinion amongst geologists. The occasion did not afford time 

 to enter into the details of the question, and the speaker therefore con- 

 tented himself with an expression of opinion offered with reserve. He 

 considered that more time and better data were required to make a sure 

 estimate ; nevertheless, he was satisfied that the evidence, as it exists, 

 does not warrant the extreme length of time so frequently supposed. The 

 recent alluvia covering the latest Quaternary deposits of our valleys are 

 rarely more than forty feet thick, in most cases not more than twenty 

 VOL. VII. 2 c 



