THE CHANNEL TUNNEL. 
403 
strata forming a coating over the surface of the older rocks, 
and so protecting them from the penetration of water. This 
may be the case ; but it is by no means certain that these rocks 
are always overlain by impervious clays ; often the Lower 
Greensand and the Wealden Beds are known to rest upon them, 
and the same may be the case under the Channel. In most of 
the borings which have been made to the Palaeozoic rocks, no 
water has been obtained. This is probably because the rocks, 
where thus reached, are really dry ; but it may also be that 
water occurs in them, but does not rise in the bore-holes. 
The scheme proposed by Prof. Prestwich is entirely new, and 
as yet there is not sufficient evidence to enable us fairly to 
decide as to its value. This evidence can only be obtained by 
experimental boring. Borings for this purpose will have a 
double value, for they will give important aid in deciding upon 
the probability of finding coal among the Palaeozoic strata of 
the south-east of England.* One such boring is already in 
progress in Sussex. This, the first purely scientific boring in 
England, has already yielded important practical results ; if 
continued, it cannot fail to yield more.f 
Briefly reviewing the salient points of our discussion, we 
remark that, all things considered, the Lower Chalk offers the 
best chance of constructing a tunnel under the Channel. It is 
thick, persistent, and, in itself, is practically impervious. It is 
moreover easily worked, and a tunnel taken through this 
formation would be shorter than any other. These are points 
of the greatest importance, and they are sufficient reasons for 
selecting the Lower Chalk for the work. 
Should the attempt to construct a tunnel through the Chalk 
prove unsuccessful, it may be well to try the Palaeozoic rocks. 
The same shafts will serve this purpose, if the water from the 
Chalk (which alone will hinder the former scheme) can be 
tubbed back. These shafts could be continued down to the 
older rocks, and trial headings be driven under the sea. If the 
results should be favourable, it would become a question whether 
to continue the tunnel at that spot or to select another line 
more to the south-west, where, the Palaeozoic rocks being nearer 
the surface, the approaches could be much shortened. 
Lastly, should these attemps fail, there would still remain 
the Kimeridge Clay. The shortest and most suitable line for a 
tunnel through this formation can only be determined by 
borings on the English coast. 
* Prof. Prestwich has fully discussed this question in a former number of 
the Popular Science Review, July 1872. 
t I would here like to mention that subscriptions in aid of the Sub- 
Wealden boring are urgently needed. They may he sent to Mr. H. Willett, 
F.G.S., Hon. Sec., Arnold House, Brighton. 
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