292 
ME. PEESTWICH ON THE GEOLOGY OE THE DEPOSITS 
mountain districts. What I wish to point out is the probability of the continuance of 
severe cold during the period when the high-level gravels were in course of formation, 
with, at the same time, a concurrent gradual amelioration of the climate, accompanied 
possibly by a greater rainfall, and certainly by great spring floods. 
I have before shown how impossible it would be for the present rivers, even during 
their greatest floods, to attain a height at all approaching to the level of the high-level 
gravels ; but, taking the additional discharge resulting only from this melting of the snow, 
independently of any larger rainfall, the floods must formerly have been far greater 
than those of the same districts in the present day, and have given to the rivers for a 
portion of the year a torrential character. That the water-supply was adequate to fill 
at times the broad and shallow bid channels is evident from the facts and is borne out 
by calculation. The Waveney waters may, even now, when the valley is flooded, give a 
sectional area of, say 1400 square feet. To fill the channel of the old river, supposing 
it to be on the level and of the width of the high-level gravels, would have only 
required a volume of water of a sectional area not exceeding 7000 feet, or five times 
as large. So with the Ouse, the measure, with the valley flooded, may be 4000 feet 
for the present river, and 20,000 feet for the old postpliocene river; for the Somme of 
to-day 3000 feet, and for the old river 16,000 feet; and for the Seine 8000 feet now, 
and 36,000 feet formerly. These are merely rough approximate estimates. They will 
serve, however, to show that to fill the old channels, before the excavation of the exist- 
ing valleys, to their entire breadth, would not have required more than, if so much as, 
four or five times as much water as now flows during floods ; but it must be remem- 
bered that the normal condition of these quaternary rivers would be like that of rivers 
of the present day that are subject to heavy periodical floods and have large and wide 
channels, small portions only of which are filled by the river during a great part of the 
year, — dry sand and shingle banks then occupying the larger portion of the area. A 
supply in fact very little if any larger than that drained off by the existing rivers might 
have occupied the comparatively dry channels during the dry season, whilst these old 
channels would be filled to overflowing during the melting of the snow in spring, inde- 
pendently of any excess of rainfall, and be subject to periodical floods and inundations, 
such as now are of annual occurrence in Arctic countries, when the waters rise 40 to 
50 feet or more above their ordinary level*. 
Although I can conceive that, granting an indefinite length of time, the wearing 
power of torrential rivers might effect considerable erosions, we shall find that other 
causes have assisted to produce the immense valley-excavations we are now contem- 
plating. For, if the period is assumed to have been one of severe winter cold, we must 
follow out the consequences of that assumption, not only with regard to the floods 
following upon the winter snows, but in all its collateral bearings. 
The effect of the freezing of the rivers and the transport by ice of the boulders, 
gravel, and organic remains lying on the shores, has already been discussed. In addi- 
* The periodical rains of tropical countries produce a somewhat similar but smaller result. 
