March 16, 1882 | 
logical chart of the straits was thus made, showing the 
outcrops of the beds in the Channel from the Gault up- 
wards. On this chart the submarine geology shown on 
the map (Fig. 1) accompanying this article is founded. The 
outcrop of the Gault is shown, and an approximate separa- 
tion of the Lower Chalk from the Upper Chalk. It may 
be mentioned here that the lowest division of the chalk 
made by the French geologists, and known as the Craie de 
Rouen, does not exactly correspond with our Lower Chalk, 
which includes part of the overlying sub-division, the Craie 
Moyenne. The series of vertical sections (Fig. 3), which 
- has been constructed to illustrate the remarkable persist- 
ence of the subdivisions of the Chalk, partly shows also 
the difference between the English and French classifica- 
tions. The base of the Gault has been selected as an 
artificial datum line in plotting these sections. 
The Lower Greensand on the other hand is probably 
represented only in its upper beds in the Boulonnais, 
while the Wealden beds are so changed and attenuated 
that the subdivisions made in England are unrecognisable 
in France. The Kimmeridge Clay reappears, but much 
reduced in thickness, as indeed is the case with all the 
lower secondary formations. For all these earlier beds 
rest upon an uneven rock-floor, carved out of a vast mass 
of contorted palzozoic rocks; and a ridge forming a pro- 
minent feature in this old surface existed in what is now 
CALAIS 
ARTESIAN WELL 
LONDON 
MEUX&C°S BREWERY 
KENT COAST 
SECTIONS 
oy : eer 
© 100 200300 400 500 600 700 800 
NATURE. 
PALZ0Z0IC 
Rocks 
465 
the north part of the Bas-Boulonnais, and perhaps stoo 
above water through all the earlier part of the Secondary 
Period, until it was finally submerged beneath the water 
ofthe Gault sea. The filling up of the inequalities in the 
old surface probably contributed to the more even distri- 
bution of the Gault and Chalk. The only point at which 
the Paleozoic Rocks now appear at the surface in the 
district is in the north-east corner of the Bas-Boulonnais 
but they have been reached in boreholes in London 
Calais, and Guines as shown in the sections. 
It was proposed by Prof. Prestwich to carry the tunnel 
through these Palaeozoic rocks on the grounds that they 
are of great dimensions, and protected by overlying im- 
permeable strata. But their great depth has prevented 
much attention being paid to the scheme; at Calais they 
are at 1160 feet below the surface, in London 1064, and 
near Battle in Sussex they have not been reached at 1900 
feet. It has also been suggested that a tunnel starting in the 
Weald Clay on the English might be carried through into 
the Kimmeridge Clay on the French side without encoun- © 
tering the intervening Portland beds, it being supposed 
that these watery strata might thin out and leave the two 
clays in contact. But at the present time the inquiry has 
narrowed itself to the Chalk, the lower part of which is not 
only most suitable for tunnelling, but has the advantage 
of occupying the narrowest part of the Channel. 
=| TERTIARY. 
POS] STRATA &° 
ST.MARGARETS JES GUINES. 
BOREHOLE ——a |. BOREHOLE 
S2MILES.S.OF CALAIS 
SANGATTE BLANC NEZ 
BOREHOLE SECTION 
UPPER 
CHALK 
ab as LOWER 
ALE 
2. CHALK 
{ GAULT. 
-:LOWER GREENSANIE 
PAL0ZOIG 
ROCKS 
FEET 
Fic. 3.—Sections showing the Persistence of the Lower Chalk. 
The two schemes which are offered for tunnelling 
through the Chalk may be briefly stated as follows :— 
1. The Channel Tunnel Company, with which Sir 
John Hawkshaw is ccnnected, proj ose a tunnel starting 
from Biggin Street, Dover, with a gradient of 1 in 8o, | 
passing under the north spur of Dover Castle Hill, and 
thence continuing to a Point on the shore known as the 
Fan Hole at a distance of 2 miles, 4 furlongs, 2°50 chains 
from Biggin Street, and at a depth of 115 feet below high 
water ordinary spring tides. From this point it will run 
across the straits to join the French tunnel, which com- 
mences near Sangatte, and as Sir ]. Hawkshaw has always 
advocated a straight line of tunnel, we presume that such 
will be the case here. 
2. The Submarine Continental Railway Company, with 
which Sir E. Watkin is associated, propose a tunnel con- 
nected with the South-Eastern Railway, about two fur- 
longs west of Folkestone entrance of the Abbotscliff 
Tunnel by a tunnel descending at a gradient of 1 in 52 to 
the bottom of the No. 2 Shaft, near the west end of the 
Shakspeare Tunnel, at a depth of 126 feet below high water 
ordinary spring tides. From this point the tunnel will 
continue for about a mile towards the head of the pier 
in a direction slightly diverging from the shore, and 
finally curving round, will fall into the line of the French 
tunnel near Sangatte. In Sir John Hawkshaw’s scheme 
the tunnel will start on the English coast in the lower 
part of the Upper Chalk, but will rapidly reach th 
Lower Chalk, and probably remain in it throughout. In 
that proposed by Sir Edward Watkin, the inclined plane 
leading to the tunnel, and the tunnel itself, will be driven 
from end to end in the same bed of grey chalk which 
constitutes the lower part of the subdivision of the Lower 
Chalk. 
So far as our information goes at present it seems that 
the most serious obstacle will be water, and it is therefore on 
their relative liability to flooding that the proposed tunnels 
must be judged. In the Belgian Coal-field, where Coal- 
measures are worked under secondary strata full of water, 
it is found that those works are dry which “follow the 
vein,” while those which cut across the strata are invaded 
by water. For the Coal-measures rise up to and end off 
in succession against the base of the secondary strata 
(which rest unconformably upon them), so that every bed, 
whether porous or not, is in contact with the water. It 
is clear then that a level, which cut across the strata, would 
run a far greater chance of intersecting a water-bearing 
rock than one which always followed the same bed. Now 
| a precisely analogous position is held by the rocks which 
form the floor of the Channel relatively to the water 
which occupies it. The strata rising to the south-west 
crop out in succession in the sea-bottom, and, being for 
the most part bared by the scour of the tide, must be fully 
saturated with water. In an impermeable bed this would 
