398 
POPULAE SCIENCE BEVIEW. 
project for a tunnel which runs the slightest risk of encountering 
any of these formations should be entertained. 
The Weald Clay, on the English coast, is from 300 to 400 ft. 
thick ; for the most part it is an impervious clay, well adapted 
for tunnelling, though it often contains beds of sand which yield 
water. But, however suitable for tunnelling the Weald Clay 
may be on the English coast, it does not exist at all as a recog- 
nisable division on the French coast, and therefore it, too, must 
be discarded. 
The Purbeck Beds (formerly known as the Ashburnham Beds) 
come to the surface on the north-west of Battle, and in them 
the Sub-Wealden boring begins. Altogether they are about 
300 ft. in thickness ; they consist chiefly of shale and clay, but 
contain some beds of sandstone and limestone. The sandstones 
almost certainly, and the limestones probably, would yield 
water ; * but here again, as with the Weald Clay, the strata 
occur only on the English side of the Channel, and we have no 
means of knowing how far under the bed of the Channel they 
may extend. We have thus successively discarded all the strata 
which lie between the Gault and the Kimeridge Clay ; all be- 
tween these formations are either inconstant or are unsuited for 
tunnelling. 
The Gault is a very stiff and impervious clay, with very few 
partings or divisions, and none of those of any consequence ; 
without much doubt, it passes regularly across the Channel, and, 
but for its thinness, would be better suited for tunnelling than 
any other formation. But on the English coast it is only about 
100 ft. thick, whilst on the French coast it can scarcely be more 
than 50 ft. Along its outcrop on the north side of the London 
Basin, and occasionally when pierced in wells, the Gault is 
found to be of greater thickness than is here stated, and it has 
been suggested that probably the thickness has been wrongly 
estimated on the shores of the Channel.! There is, however, 
no good reason for supposing that such is the case. Although no 
well near Folkestone has been carried entirely through the 
Gault, its thickness is well known. Mr. F. G. H. Price has 
lately published a careful description of the “ Gault of Folke- 
stone ; ” and he, from actual measurement, makes the thickness 
to be 99 ft. at Copt Point.! No exact measurement has yet 
been made at Wissant, but the Gault is certainly thinner there 
than at Folkestone ; it has been proved to be so in borings at 
Calais and near Guines. 
Even if the Gaidt were much thicker than it is, there would 
* Some springs were tapped in the Sub-Wealden Boring. 
t See especially the remarks by Mr. Homersham in the discussion on 
Prof. Prestwich’s paper. 
X “ Quart. Journ. Geol. Soc.,” vol. xxx., p. 342. 
