duree de l’epoque quaternaire, et je crois que les calculs qu’on 
a faits sur ce point sont purement hypothetiques.”* 
The language of the careful editor of Reliquice Aquitanice, 
Professor Rupert Jones, is an echo of many others. He says, 
“ How long a time was required for the changes in land and 
sea, mountain and valley, for the change from the glacial to 
a boreal and pluvial climate, with its ever-recurring snow and 
rain, excavating the higher valleys and filling up the coast 
valleys with enormous accumulations of sands and gravels, we 
have but few means of calculation to judge by.” f 
With the Lyellian school, the theory of the formation of the 
Somme and Thames valleys, and of all other valleys in whose 
flanks or basin palteolitliic implements have been found, is, that 
a flat surface of chalk was left by the original sea, here and 
there dotted with banks of marine tertiary mud and sand ; that 
the action of the rain gradually formed hollows, and connected 
these, until a channel was made, deepened by ordinary rains 
and floods; and that the waters occasionally accumulated, so as to 
erode the chalk and distribute the pebbles as we now find them. 
It is admitted, — nay, supposed, — that, according to the 
calculations of Mr. Croll, this erosion would take place at first 
only at the rate of 1 foot in 1,000 years, and afterwards some- 
what more rapidly in the limited area of the valley. The 
Thames now lowers its bed only 1 foot in 11,740 years, and 
therefore the amount of time since the deposit of the gravel- 
beds at Gray’s Inn or at Ealing, say 100 feet above the present 
level, and four miles wide, is truly inconceivable. 
Now, as we are not dealing with a fact of observation, but of 
deduction, if it is inconceivable, it is, of course, relegated to 
the domain of the imagination. The action of rain and rivers, 
though a true cause, ceases to be a true cause, in relation to an 
effect which it cannot produce. With any amount of time and 
present forces, the work assigned is plainly impossible. The 
eroding and lifting power of the present streams are wholly 
inadequate. On the one hand, Sir C. Lyell says, “ I see no 
reason for supposing that any part of the revolutions in physical 
geography, to which the maps above described have reference 
(post-pliocene oscillations of level), indicate any catastrophes 
greater than those which the present generation has witnessed.” 
But, on the other hand, Professor Prestwich lays it down, 
“That the formation of the higher gravels can be owing to the 
action of the present rivers is clearly impossible under existing 
* La Seine, Belgrand, p. 103, Introduction, 
t Proceedings of Geologists' Association, vol. iii. p. 207. 
