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are, however, merely relative terms ; no formation can be ex- 
pected to be absolutely impervious for a long distance, and 
it becomes merely a question as to whether our methods of 
pumping can keep under the water likely to be met with. 
Having now stated the main conditions of the question, we 
will proceed to examine the actual geological structure of the 
Straits of Hover, so far as this is known, in order to ascertain 
what is likely to be the arrangement of the strata along the 
lines of the proposed tunnels. 
Sufficient information for our purpose is given on Plate 
CXIV., and if the arrangement of the rocks on the surface (as 
shown in the map) be compared with the arrangement beneath 
the surface, along certain lines (as shown in the sections), a 
very fair idea of the geological conditions will be obtained. 
The lowest bed yet known on the English side is the Oxford 
Clay, which has been reached in the Sub-Wealden Boring. 
Above this come the Kimeridge Clay and the Portland Beds, 
none of which appear on the English coast of the Channel 
until we get as far west as Dorsetshire. As yet we know little 
of the underground range of these beds to the north, but of the 
higher beds we do know something. These all dip gradually 
to the north (as shown in fig. 4), each passing regularly under 
the one next above it, until we reach the Chalk, which is the 
highest English formation with which we are now concerned! 
Underneath the Chalk the Gault will everywhere occur (in 
the area in question) ; wherever the Grault is exposed at the 
surface we also know that the Lower Greensand will occur be- 
neath it; and, again, beneath the area coloured as Lower Green- 
sand on the map, we shall certainly find Weald Clay. But 
it does not follow, because we thus get a complete succession 
along the outcrops, that therefore the succession is complete 
at any one place. In sinking a well, for instance, near Dover, 
we should reach the Gault ; perhaps also the Lower Greensand ; 
but we might not find the Weald Clay, or the beds below it ; 
probably we should come at once upon the Palaeozoic rocks. 
In the section (fig. 4) this incomplete succession is shown, 
but here it is a matter of inference only ; we have, as yet, no 
absolute proof that certain beds are wanting under Dover. 
In the section (fig. 3) which shows the structure along the 
French coast the case is different; here we have proof that the 
general structure is as is there shown. 
The Oxford and Kimeridge Clays, and the Portland Beds, 
which nowhere appear at the surface in the south-east of Eng- 
land, occupy a great part of the ground in the Bas Boulonnais. 
They regularly pass under each other to the north-west, and are 
finally covered up by the Wealden Beds, the Lower Greensand, 
Gault, and Chalk. But we certainly know, from a deep boring 
