PEOCEEDINGS OF GEOLOGICA.L SOCIETIES. 189 
has 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 tlie valley, whilst a more connected 
series of gravel beds skirts the base of the valley. The chief portion of 
the valley is, however, occu])i( d 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 
hones and Jlint imjjlements. and a few shells as above, ir- 
regularly dispersed throughout 5 to 14 
Tliese beds repose upon a base of chalk. The site having been long oc- 
cupied as a Gallo-Eoman 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- 
cially 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, 
