
BANKS OF THE TWEED AND SOME OF ITS TRIBUTARIES. 561 
geologists who consider that these high-water marks on the banks of rivers 
indicate the levels to which the rivers rose, in former times, when in flood,— 
assuming that the rivers then flowed in the same channels which they at present 
occupy. 
Mr Tytor was, I believe, the first to suggest this view in reference to the 
River Somme and Oise in France, and the Rivers Aire and Waveney in England. 
Mr Brown also, in his paper on the terraces of the Earn and Teith, adopts 
that. theory; and Mr James Gerxreg, in his book, to which I have already 
referred, seems to adopt it, when speaking of the immense deposits of sand and 
gravel formed. by existing rivers, by reason of the far greater floods’ which 
occurred in them formerly. 
Mr Brown is of opinion, if I have read his paper correctly, that in 
the River Earn, where floods now reach no higher than 6 feet, floods im 
former times reached to a height of 57 feet, making the river-flood 57 feet: 
deep: So also it is contended that in the River Teith, the floods, which reach 
now no: higher than 6 feet, im former times reached to a height of 24 feet, 
forming a stream more than 24 feet deep. 
Mr Tytor, applying that view of the matter to the Somme, a river 
apparently about the size of the Earn or the Teith, maintains, that when the: 
high level gravel bed was laid down by the river,—the river occupied its present: 
channel! 84 feet below that gravel bed; and, therefore, when that gravel bed 
was deposited, it must have been by a flood 84 feet deep at least. 
These views, assumed: to be correct, are adduced as evidence in favour of 
glaciers and: other forms of land ice, the melting’ of which in summer, it is 
said, would give: rise to these enormous floods. In support of these views, 
suggestions: have been made regarding larger amounts of rain which may have 
fallen in: the country, owing to a more rapid condensation of vapour on the ice 
or snow-capped hilis. 
I cannot discover good grounds for these views. Mr Prestwicu, who has 
carefully studied the physical features of the River Somme with reference to 
this theory, and for many years devoted himself to what. are called the Drift 
deposits, says, that if Mr Tyitor’s views are correct, the quantity of water 
required to raise the: Somme to the height of the highest river gravel beds would. 
be 100 times more than:any flood ever known to have occurred in the river, 
and, he cannot see where such:a supply of water could be obtained. 
No: doubt, it: may be said, that we are not. entitled to reject any geological 
theory, if it may possibly be correct; but if a more: probable explanation can 
be suggested—that explanation surely should be preferred. 
The: view which I have submitted in this paper, seems to rest on stronger 
grounds, than there being merely in its favour a larger body of evidence. I 
think that, if the existence’ of old sea margins, up to even’ the smallest heights 
VOL. XXVII. PART IV. 71 
