CONTAINING FLINT IMPLEMENTS, AND ON THE LOESS. 
301 
The next possible standard of measure is the time required for the excavation of the 
valleys themselves. I have already described the agents which probably cooperated in 
this gigantic operation. That it must have been one of great time there can be no 
doubt ; but the like operations at present in progress by no means furnish us with the 
gauge of the rate of the denuding action. In considering this point there is, besides 
the greater floods and severer cold, another element which must not be overlooked. 
This is the varying solvent power of spring- and river-waters. This there is reason to 
suppose may be greater in cold than in temperate climates, for Agassiz has shown 
that fallen snow holds excessive proportions of air in combination ; so that, during 
inundations resulting from the melting of snow in spring, the flood-waters becoming 
loaded with soil and vegetable matter must necessarily have presented conditions 
favourable for generating carbonic acid in large quantities, with which the ice-cold 
waters would become highly charged. Thus the solution, both of the calcareous beds 
forming the river-channel and of the strata perforated by the gravel and sand pipes 
just alluded to, may have been accelerated much beyond any effects now observable in 
these districts from the present action of ordinary spring- and river-waters. 
An indication of time-measurement, which has been often referred to in relation 
both to the lapse of time and its late date, is the formation or excavation of the 
British Channel between the South-east of England and the opposite coast of France. 
The grounds on which it has been inferred are, the identity of the strata on the two 
sides of the channel, and the community of the fauna and flora. This to a certain 
and great extent is true, and there can be no doubt that the severance of the two 
countries took place at a comparatively late geological period ; but that it was the last 
change of all I am not prepared to admit. In fact the question has been treated in its 
immediate application in a manner purely hypothetical. The geological evidence of the 
substrata has been constantly had in view, whilst that of the superficial postpliocene 
beds, which relate directly to the period under consideration, has not been attended to. 
Whether or not there may have been a break between the two countries at the high- 
level valley-gravel period I could not say with certainty. We have evidence of these 
beds occupying, on or near the coast-line, a level of from 50 to 100 feet above the sea 
on both sides of the channel. This may arise from the sea encroaching on the land 
and so intersecting, at varying distances from the old line of coast, the planes of the 
old river-channels — which, like the present river-beds, necessarily slope from certain 
heights inland to the sea-level, — or from an elevation of the land. The evidence, pro- 
bably, is in favour of the operation of both causes. The difference of height between 
the fossiliferous high-level gravels at Amiens and at Abbeville is 60 feet. This is in a 
distance of 28 miles. If we prolonged these beds at Abbeville, where they are 96 feet 
above the sea, on the same plane sea-ward, they would reach the level of the sea at a 
distance of 45 miles below Abbeville, or 29 miles beyond the present coast-line. The 
same measurements applied to the high-level gravels of our own coast give nearly similar 
results. If this be correct, a sea-channel, although very contracted, may then possibly 
