FROM THE CHALK OF HERTFORDSHIRE. 
145 
scarcely 20 millions, which is very little, if any, more than the 
Colne, a few years before this, had been found to carry away daily ,* 
the amount available for discharge along the coast, if it could get 
as far, thus being reduced to an almost inappreciable quantity. The 
absurdity of Mr. Homer sham’s conclusion is manifest when we con¬ 
sider that it implies a flow of water under Watford, and in some 
inexplicable manner into the sea, equal to the mean flow of the 
Thames about midway between Staines and Teddington. 
We have next to follow, as nearly as we can, the course of the 
water which percolates into the chalk. It descends to the plane of 
saturation with very varying degrees of rapidity ; where the chalk 
is compact, the percolation is exceedingly slow ; where it is fissured 
or there are “pipes” filled with loose gravel which have been 
formed by the dissolution of the chalk in the manner already ex¬ 
plained, it may be very rapid. In our Upper Chalk district the 
water on its way down encounters nearly horizontal bands of flints 
loosely aggregated together, along which it flows much more readily 
than in the compact chalk. It will therefore tend to flow along the 
plane of dip of the strata, which, in the valleys of the Lea and 
Colne, is from north-west to south-east at a slight angle from the 
horizontal. 
From sections exposed in our chalk-pits we know that “pipes” 
are more or less vertical, and therefore while some of the water is 
carried in the direction of dip, which is nearly horizontal, some of 
it rapidly sinks in a nearly vertical direction, and as the water flows 
along the plane of dip it will be constantly encountering these pipes 
and be carried down in them. Towards its base the Chalk is more 
retentive of water and less fissured than it is throughout the greater 
part of its mass, so that the percolation becomes gradually slower, 
or rather would do so if the rock were not here, when in a natural 
condition, already fully saturated. Below the Chalk the great com- 
paratively-impervious mass of the Gault Clay holds up the water as 
in a but very slightly porous basin ; in fact we may, for all practical 
purposes, altogether neglect its porosity, and consider that the water 
in our Chalk is retained naturally by the Chalk Marl and Gault at 
its base as efficiently as the water is retained artificially by the bed 
of a well-made reservoir.f 
The term “underground reservoir,” as applied to the water in 
the Chalk, does not convey a clear idea of the actual state of satu¬ 
rated rocks. We naturally picture a reservoir as a sheet of water 
which maintains a horizontal surface alike when it is added to or 
taken from, but chalk offers a very considerable frictional resistance 
to the flow of water in it, and it is a long time before water added 
at any one spot percolates laterally through it so as to assume a 
horizontal surface; in fact it very seldom if ever does so for any 
great distance. The resistance chalk thus affords to the passage of 
* Stephenson’s ‘ Report to the London and Westminster Water Company,’ 
1840, p. 12. 
f I have omitted any reference to the Palaeozoic rocks under the Chalk, having 
previously treated of them in their relation to the 'water-bearing strata. ‘ Trans. 
Watford Nat. Hist. Soc.,’ Yol. II, p. 241. 
VOL. VI.—PART v. 
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