176 



THE GEOLOGIST. 



size of the glaciers of this epoch, it may be inferred that the land did 

 not attain the altitude it had during the preceding (subaerial) one. 

 Possibly the land, 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, YV 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 Galvvay would, under the conditions 

 named in the text, form an upland plain. From all I can observe, the district around 

 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 first 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., are 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 Du Noyer in his highly 

 interesting paper, published in the c 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 been 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 iu 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 Rev. Mr. Symouds correctly observes that 

 the physical geology of the oldest or highest of the Somme valley gravels "proves their 

 immense antiquity even more than do their fossil remains." (Geolocw of the Kailwav, 

 p. 14.) 



