466 
make little or no difference to mining operations, but to 
encounter a porous bed such as the Lower Greensand 
under such circumstances would be to tap a reservoir as 
inexhaustible as the sea itself. Fortunately an excellent 
water-tight barrier between this quicksand and the Chalk 
above is provided in the Gault. The Upper Greensand, 
which lies next above the Gault, is generally thin, but 
occasionally expands rapidly from almost nothing to six 
or eight feet, and, though as rapidly dwindling down 
again, it might under such circumstances conduct a large 
amount of water. The French engineers have tested it at 
intervals by borings in the drift-ways, and have observed 
that water rises from it in most cases. But it is generally 
too insignificant to prove dangerous. 
That it would be advisable to choose the most favour- 
able bed, and to follow it for the whole distance, if this 
were possible, is not to be denied. But a study of the 
map shows that to do this it would be necessary either to 
make at least two considerable curves in the tunnel, or- 
else to make it rise and fall with the beds, which would 
probably be prejudicial to drainage and ventilation. For 
the form of the outcrop of the Gault shows that close to 
both shores the strata are bent upwards in the form 
known to geologists as anticlinals. The outcrops in- 
cline to the north-east in such a way that the Gault occu- 
pies a position in the line of prolongation of the Lower 
Chalk, indicating clearly that it is arched upwards at 
these points, and therefore intersects the sea-bottom 
further to the north-east than it would if it formed a 
plane surface. The anticlinal which passes near San- 
gatte is partly shown in one of the longitudinal sections 
accompanying the map, but would be more manifest 
in a section running in the direction of the proposed 
tunnel. Again, about midway between Dover and Folke- 
stone an isolated patch of Gault appears through the 
Lower Chalk. This patch is believed to be bounded on 
the south-west by a fault, not necessarily of large amount, 
and probably dying out to a mere gentle roll in the strata 
seaward. To make the lie of the beds more intelligible, 
the French engineers calculated the position of three 
horizontal lines following the top of the Gault at a depth 
of 50, 100, and 150 métres respectively below low-water 
mark, and one of these has been placed upon the map 
accompanying this article. The inequalities of the sur- 
face of the Gault are shown by this line in the same way 
that the inequalities of the ground are shown bya contour 
line. 
The thickness of the Lower Chalk is such that in spite 
of these undulations in the beds a tunnel originally 
planned to run near the middle of the subdivision, might 
continue a straight course with an unchanged gradient, 
and still remain with its limits. But if running near the 
top a synclinal or downward bend would bring the Upper 
Chalk into the line of the tunnel, while if running near 
the bottom, an anticlinal or upward bend would bring in 
the Gault. It is obvious then that a tunnel traversing the 
lower portion of the Chalk, on encountering an anticlinal, 
would be compelled to bend towards the north-east in 
order to preserve a constant gradient, and at the same 
time avoid touching the Gault. It is believed that the 
route advocated by Sir E. Watkin is so planned as to 
round the two anticlinals described above, as occurring 
on either side of the Channel. It remains to be seen 
whether the form of the beds will admit of its being 
carried in a straight line, and at the same time iu the 
bottom beds of the Chalk, for the remainder of the 
distance. In the route proposed by Sir John Hawkshaw, 
the effect of an anticlinal would be to bring up the lower 
beds of the Chalk into the line of tunnel, and would there- 
fore not be unfavourable ; a synclinal might probably 
bring down the Upper Chalk with flints, which, as will be 
shown subsequently, is not desirable. 
A similar source of uncertainty exists in the possible 
occurrence of faults, that is, lines of fracture in the strata, 
NATURE 
Mit oar te aaa tee “ae 
[March 16, 1882 
on one side or other of which they have been vertically 
shifted. The Gault in mid-channel being presumably — 
about 50 or 60 feet thick, a fault of this amount might 
bring the base of the Chalk and the top of the Lower 
Greensand face to face. The submarine mapping, how-— 
ever, if it can be trusted, shows that it is very improbable ~ 
that there is a fault of this amount in the channel. A 
fault would displace all the beds of the Chalk in succession 
equally or nearly so, but its effect would be less percep- 
tible higher in the system where chalk would be thrown © 
against chalk, than at the base where chalk might be 
thrown against the Gault or Greensand. It remains to be 
considered which of the subdivisions of the Chalk is the — 
most suitable for operations, and whether sufficient differ- 
ence exists between the various beds to make the experi- 
ence of the Belgian miners directly applicable to the 
Channel Tunnel. 4 
The lower part of the Grey Chalk, the lowest sub- 
division of the Chalk as shown in the table, is uni- | 
versally stated to be very impermeable. Prof. Prestwich, 
in the “ Water-bearing Strata of London,’ p. 63, states : 
“ The lower beds of the Chalk are generally soargillaceous 
that the rock often puts on the character of an argillaceous 
clay, which on exposure to air and water softens to a 
tenacious mud. When first exposed its colour is bluish 
grey, but it becomes white or nearly so as it dries... . 
They everywhere form a generally impermeable mass of 
strata, between the Upper and the Middle Chalk above, 
and the Upper Greensand below.” The Grey Chalk or 
Craie de Rouen, about 60 to 70 feet from its base, — 
throws out springs. The powerful spring of Lydden 
Spout, for example, is thrown out at about this horizon, — 
where a small fault, probably the continuation of that— 
inferred by the French in the Channel, slightly disturbs 
the beds. Springs are observable also on the French ~ 
coast between Sangatte and Wissant at this horizon. 
The compact nodular Chalk is very impermeable; Prof. 
Ed. Hébert (Bull. Soc. Geol. de France, Ser. 3, t. iv. 
p. 60) states that the highest part of the Chalk with- ; 
out flints is preferable as regards impermeability to 
the Craie de Rouen itself; it has never yielded water in 
any of the Artesian wells near Paris, and is always clayey 
when met with at great depths. The various sub-divisions 
which constitute the Lower Chalk form the undercliff from 
about two miles east of Folkestone, as far as the South 
Eastern Railway Terminus. Dipping gently down to 
the north-east, they were reached at 249 feet below sea- 
level at St. Margaret’s Bay in Sir John Hawkshaw’s 
borehole.* 
The divisions of the Upper Chalk, namely the Chalk 
with flints, and the Chalk with few flints, differ chiefly in 
the mode of occurrence of the flints; in the former they 
occur in layers, in the latter they are scattered and far 
between. This difference is important, for the layers 
of flint often give rise to channels by which water cir- 
culates in the chalk. The chalk with a few flints occu- 
pies the beach from Dover Castle eastwards, until it is 
succeeded in due course by the Chalk with layers of 
flints. The former throws out a few springs, the latter a 
large number, and it is generally agreed that the Upper 
Chalk, and particularly the chalk with layers of flints, is 
the most heavily. watered part of the formation. bea: 
The circulation of water in chalk takes place princi- 
pally by means of fissures, the unfissured rock being of 
such close texture as to be practically impermeable. Prof. 
Prestwich, for example, has estimated the relative per- 
meability of chalk and coarse sand to be as 1 to 6400, 
But when the fissures are numerous, and the rock con- 
tains layers of flints, the water is enabled to circulate with 
great freedom. The Upper Chalk is more liable to fissures 
owt the ae “ele ~ inion 
- *% 
| than the argillaceous lower division, and for this reason 
excels it as a water-bearing formation. 
® See sections (Fig. 2) accompanying the map. These have been copied with 
slight m difications fad a paper by Mr. Topley in the Quarterly Yournal 
| v7 Science for Apal, 1872. 
