220 Geology of the Straits of Dover. (April, 
there would be no tendency for the joints to increase in width. 
They would probably be closed cracks, not always easily 
detected, even by a tunnel intersecting them. In a similar 
manner we would not expect to find openings or small 
caverns in the Chalk, such as well-sinkers sometimes meet 
with ; for these also are caused by the incessant passage of 
water wearing away the chalk more along certain lines than 
others. 
There is one other point connected with this subject which 
may be noticed here. It was said above that a deep well sunk 
through the Chalk would certainly meet with the bed next 
below it, but might not pass through the whole series of 
rocks if continued to a greater depth. The reason of this is, 
that under the north of France and the south-east of 
England there is a ridge of Palzozoic rocks, a part of 
which is exposed in the Bas-Boulonnais. It has been 
found in the deep well at Calais, and in wells at Harwich 
and Kentish Town. So far as is known the Chalk series 
is always perfect above it; some of the Gault is also 
found, but below that we may come directly on to the older 
rocks; or there may be a thickness, great or small, of the 
Lower Greensand and underlying beds. 
As this ridge of old rocks is always, as far as is known, 
well below the Chalk, it is almost certain that a tunnel 
driven through the Chalk would not meet with it. But a 
tunnel driven through the underlying beds might do so. It 
is not at all likely that such would be the case, for on the 
French coast there isa good thickness of Oolitic strata above 
the old rocks. Apparently these old rocks are more deeply 
covered up by secondary strata as we pass westwards. 
But if the Palaozoic rocks were met with in the tunnel 
it would probably not cause any inconvenience in the work. 
Perhaps it might turn out to be a verv great advantage ; for 
where these rocks have been reached in wells no water has 
been obtained. It may be that these rocks are destitute of 
water, certainly the water that they contain, if any, does 
not rise in the wells; and hence the wells and borings 
referred to have failed as sources of water-supply, although 
the information they give is of great interest and importance. 
It is not likely that any tunnel crossing the Straits of 
Dover would meet with these old rocks; yet, as they are 
known to underlie the Chalk at no great depth, they may 
prove of great service; for if the shafts, which will be sunk 
on both coasts in starting the tunnel, were carried down 
well into the old rocks, the water might pass from the 
tunnel, down the shafts, into these old rocks, and there 
disappear. 
