176 
THE GEOLOGIST. 
size of the glaciers of this epoch, it may be inferred that the land did 
not attain the altitude it liad during the preceding (subaerial) one. 
Possibly the laud, as indicated by the depth of the second terrace, never 
rose more than 700 feet beyond the level it stood at when the "Bidden- 
ham flint-implement gravel " was in course of formation, that is, about 
the present relative level of land and sea. Probably the field-glaciers 
were wholly, or to a considerable extent, melted in the summer 
months, thereby giving rise to extensive land-floods. According to 
this view, the rocks of the then upland plains [enormously abraded 
in the first (subaerial) epoch] would again be ground down by 'glacial 
action in winter;* while in summer, the torrents of water arising 
from melted ice would sift the glacier debris, piling its blocks, peb- 
bles, and sand into banks (Escars) and mounds, and sweeping the 
mud into adjacent lakes and seas. This last glaciation would remove 
or destroy many of the marine deposits formed in the "subaqueous 
epoch:" hence their scarcity. The Lancashire and Warwickshire 
shell-deposits have escaped its destructive power. 
I quite agree with Prestwich in referring the flint-implement gravels 
of the Seine and the Somme to two distinct epochs, between which 
great physico-geographical changes took place. f The oldest (or 
highest) of these gravels appears to have its counterpart in that 
occurring near Biddenham. I am not able to correlate any British 
flint-implement gravels with the youngest (or lowest) occurring at 
Menchecourt and other French localities ; but it is highly probable 
that some of those recently discovered by Prestwich, Evans, AV hitaker, 
and others are the equivalents in geological time of those forming the 
lowest series at the mouth of the Somme. 
* The flat country lying between Dublin and Galway would, under the conditions 
named in the text, form an upland plain. From all I can observe, the district arouad 
Galway has been covered with subaerial drift (formed during the first glacial epoch) of not 
less than 300 feet in thickness ; 1 find patches of it lying at a height equal to the number of 
feet mentioned at Tonabrocken. Much of this drift, in my opinion, has been considerably 
worn down completely or swept away, at the close of the lirst and in the middle of the 
second subaerial epochs. On the worn-down undulating surfaces of this old drift, erratic 
blocks of granitic rocks, etc., arc common : these, I consider, have been transported by, 
and dropped from, icebergs during the subaqueous epoch of the glacial period. Much 
information on the subject of Irish drifts and erratics is given by Da Noyer in his highly 
interesting paper, published in the ' Geologist ' of last July. Notwithstanding its ori- 
ginal great thickness, the singular fact is common around Galway, that in no case where 
the limestone has beeu denuded of the subaerial drift do the wide cracks and joints pre- 
valent in the rock contain a particle of it : one would have imagined the very contrary. 
I can only account for its absence on the view, advanced in the text, that the limestone, 
on which the drift was originally deposited, has, in addition to the degradation which it 
underwent towards the close of the first (subaerial) epoch, been much abraded by the 
winter glaciation of the third (subaerial) one; thereby exposing beds which were not up- 
permost when the drift was in course of deposition. 
t Apparently Mr. Prestwich does not go so far in the views stated as I do. All the 
evidences bearing on the physico-geographical changes noticed in the text show that the 
intervening time, between the epochs to which the two scries of flint-implement gravels 
arc referred, was of enormous length. The Eev. Mr. Symonds correctly observes that 
the physical geology of the oldest or highest of the Sonmie valley gravels " proves their 
immense antiquity even more than do their fossil remains." (Geologv of the Kaihvay, 
p. 1-1.) 
