218 Geology of the Straits of Dover. (Apri, 
done; the bed is too thin for this on the French coast. 
‘There is no other bed of clay which can be chosen, for no 
other goes right across the Channel. The Weald Clay of 
the English coast disappears, or else vastly changes its 
character before reaching the French coast. The Kimeridge 
Clay of the French coast may not pass beneath the English 
coast, and if it does so it would be at too great a depth to 
be of use. Moreover, these clays contain beds of sand, 
which if tapped by the tunnel would be, to say the least, 
exceedingly troublesome. 
The Chalk now remains to be considered. This, there is 
no reason to doubt, passes evenly beneath the Channel from 
shore to shore, and throughout it may be expected to 
maintain its usual characters, subject only to the decrease 
in thickness already noticed, The Chalk is well known to 
be a water-bearing bed; all the deep wells near London get 
their water from it. But in this respect it is not all alike.* 
The Upper Chalk is more pervious to water than the Lower 
Chalk, and in that the water does not pass at all readily 
through the mass of the chalk, but runs along joints and 
fissures, and especially along the lines of flint. In sinking 
wells through the Chalk it is frequently found that water is 
got at some of the bands of flint, whilst the intermediate 
bands of Chalk are either without such water, or else allow 
it to pass much more slowly. 
In the drainage works at Norwich it was found that water 
percolates slowly through the re-arranged chalk which occurs 
there, but ‘“‘in the hard and undisturbed chalk the water 
sooner disappears, although in larger quantities, through 
the various joints and fissures, and along the horizontal 
beds of flint, until it reaches the level of saturation.” 
The following paragraph is of sufficient importance to be 
reprinted here : t— 
‘““The most important point in the sewerage works, as 
bearing indirectly upon the excavation of the proposed 
Channel Tunnel, is the fat that the chief difficulties 
met with in engineering operations occurred beneath the 
river level. Here the chalk and marl (re-arranged chalk) 
are thoroughly saturated, and, at only 20 feet below the level 
of the water in the river, it was found necessary to close- 
timber the top and sides of the tunnel before the brickwork 
could be put in, on account of its giving way by the pressure 
of water. The springs under the head of only 20 feet also 
* These and similar questions are fully discussed by Mr. PresTwicuH in his 
“ Water-Bearing Strata of London,” 1851; pp. 57 to 74, and 134 to 142. 
+ J. E. TayLor and A. W. Morant: “The Water-Bearing Strata in the 
Neighbourhood of Norwich. ‘ Geological Magazine,” vol. vil., p. 121, 1870. 
