PROCEEDINGS OF GEOLOGICAL SOCIETIES. 189 



lias formed a relative measure of time, on land the extent of denudation 

 resulting from the removal of a portion of that matter supplies an obverse 

 scale. In the former case the lapse of time is chronicled by constantly- 

 accruing deposits, whereas in the latter case the deposits cannot exceed a 

 certain thickness. They are constant quantities, and their dimensions are 

 no measure of their age. The only test of their age consists in their or- 

 ganic remains, and in the depth of the valleys below the terraces on which 

 portions of them are lodged. In speaking of river action, the author does 

 not refer to the slow and sluggish streams of this country, but to the more 

 active streams of countries of greater rainfall, or to old conditions of 

 former periods. 



Mr. Prestwich then proceeded to refer to a large pictorial section of the 

 celebrated pit at St. Acheul, near Amiens. The artist had not visited 

 Amiens, but had skilfully contrived to give a sufficiently accurate repre- 

 sentation of the town and valley, for the purpose of showing the general 

 relation which the ground there bore to the surrounding district. The 

 details of the pit were, however, all given from actual survey by the 

 speaker. The surface of the ground at the pit is 100 feet above the level 

 of the Somme, which flows in the valley at the foot of the hill. The 

 valley itself is about one mile broad. The hills on either side, rising to a 

 height of 200 to 300 feet, consist of chalk, with a few and distant cappings 

 of Tertiary strata. 



On platforms of various breadths, generally on the top of low hills ad- 

 joining the valley, patches of gravel occur at intervals more or less long 

 from the lower to the upper end of the valley, whilst a more connected 

 series of gravel beds skirts the base of the valley. The chief portion of 

 the valley is, however, occupied by alluvial beds, beneath which the last- 

 mentioned gravels, with their brick-earth, pass. 



The higher level gravels rarely contain organic remains. The pit at 

 St. Acheul affords a singularly good example of these beds, and is unusu- 



ally rich in organic remains, and also in flint implements. 



The section exhibits : — Feet. 



1. Brick-earth (Loess) without organic remains 10 to 15 



2. A variable bed of whitish, marly sand, with numerous 



freshwater and land shells of recent species, and a few 

 mammalian remains 3 to 7 



3. Variable beds of subangular flint gravel — some white, 



others ochreous and ferruginous. Numerous fossil 

 bones and flint implements, and a few shells as above, ir- 

 regularly dispersed throughout 5 to 14 



These beds repose upon a base of chalk. The site having been long oc- 

 cupied as a Gallo-Roman cemetery, the upper brick-earth is intersected 

 with pits and graves,— in some there are stone, or rather hard chalk, cof- 

 fins, whilst in others the nails and ironwork alone remain, the wood hav- 

 ing entirely decayed away. These portions of disturbed ground are 

 easily recognized by their darker colour, their contents, but more espe- 

 ciallj r by the break they produce in the stratification of the beds. So long 

 as the ground is undisturbed the lines of the brick-earth, the lamination of 

 the sands, and the rough bedding of the gravel are continued in horizontal 

 planes without break. Any interference from above breaks these lines 

 and mixes the different beds, and renders the disturbance at once apparent. 

 In the absence of any such indications it is to be assumed the fossils and 

 the flint implements are in undisturbed ground. 



The flint implements are found scattered irregularly through the gravel, 



