1872.] Geology of the Straits of Dover. 217 
gate Beds. An account of the great difficulties which had 
to be surmounted here is given by Mr. Simms.* The Seven- 
oaks tunnel, on the new line to Tunbridge Wells, was driven 
through the Hythe Beds, and partly through the underlying 
Atherfield Clay. The former in their lower parts were 
largely charged with water, which pours out in a fully- 
formed stream from the tunnel’s mouth. 
The tunnel which it is proposed to take from the South 
Foreland to near Sangatte will, it is supposed, go entirely 
through the Chalk-without-flints. 
The one great danger to be dreaded in driving a tunnel 
under the Channel is water, in quantities so great that it 
cannot be mastered. It is only in view of this danger that 
engineers need be much concerned with the geological 
structure of the Straits of Dover; for, supposing there were 
no great influx of water to be anticipated, it is a matter of 
comparatively little consequence whether the tunnel be driven 
through clay, sand, or chalk. 
Water may occur chiefly from two causes:—the natural 
porosity of the rocks allowing water to pass freely down 
from the sea along the bedding into the tunnel; or faults 
and fissures cutting through the beds, and thus letting in 
water. We will consider the former first. 
We have seen that two divisions of the Lower Greensand 
in East Kent are exceedingly porous (the Folkestone Beds 
and the Hythe Beds); these are naturally porous, and water 
passes freely along the bedding whether fissures be present 
or not. Any tunnel driven through them would, without 
much doubt, meet with very large quantities of water. Again, 
as regards the Wealden Beds; a large proportion of the 
Weald Clay is a stiff impervious clay, very well adapted for 
tunnelling where clay only occurs; but at various horizons 
within it we meet with beds of sand, which would most 
probably be charged with water. Still lower, and further 
out to sea, there would be the Hastings Beds, the sands of 
which are finer in grain, and perhaps less pervious to water 
than those of the Folkestone Beds, but still sufficiently so 
to make tunnelling through them a hazardous operation. 
The Portland Beds are mainly porous, whilst the Kimeridge 
Beds are mainly impervious; but in the latter series there 
are beds of sandstone which would probably yield much 
water. 
If the tunnel could be driven wholly through the Gault 
there need not be much fear of water. But this cannot be 
* Practical Tunnelling as exemplified in the particulars of Bletchingley and 
Saltwood Tunnels. 1844. 2nd edition, 1859, by W. D. HasKELL. 
VOL. Il. (N-S:) 2 
