1875.] The Channel Tunnel. 501 
following remarks extracted from a paper read by him before 
the Institution of Civil Engineers in December, 1873 :— 
** The chalk formation, which everywhere in the south-east 
of England and the north-west of France underlies the 
Tertiary series, has a maximum thickness of from 1000 feet 
to 1300 feet; but, as much of it has in this area been worn 
away or denuded before the deposition of the Tertiary strata, 
its actual thickness in the distri¢t under notice varies from 
300 or 400 feet to 800 or 1000 feet. The upper beds consist 
of almost pure carbonate of lime, easily worn by water, and 
being also soft and fissured they are readily permeable. 
But the Lower Chalk or Chalk Marl contains so large a 
proportion of argillaceous matter and silica in a state of fine 
division that some beds pass almost into a clay, and when 
unbroken and compact very little water can pass through 
them, and then only with extreme slowness, though this will 
increase under pressure. But although a small bore-hole 
or even a shaft may often be carried through a considerable 
thickness of Lower Chalk and no water obtained, the occur- 
rence of fissures is too uncertain to render it a reliable me- 
dium over a large area. In some cases where the Lower 
Chalk comes to the surface, and is more broken and fissured, 
the quantity of water it yields is very large, as in the instance 
of the Tring cutting described by Robert Stephenson, where 
the discharge was at the rate of 1,000,000 gallons per day; 
or at Folkestone, where the town water supply is obtained 
from the Lower Chalk of the adjacent downs. On the other 
hand, the Chalk Marl in France and Belgium aéts as an 
impermeable stratum in stopping the passage of water from 
the very permeable Upper Chalk into the underlying coal 
measures ; and no water was found in it either at Kentish 
Town, Harwich, Southampton, or Calais; but the diameter 
of the bore-holes by which they were traversed were very 
small. At Calais one spring was met with at a depth of 70 
feet in the Upper Chalk, and the water was brackish, show- 
ing communication with the sea. Nor must it be forgotten 
that wells in the Chalk under London have to be carried or 
bored to depths of from Io to 300 feet before meeting with 
water-bearing fissures, or else headings have to be driven in 
search of one. Again, the escarpment of the North Downs 
and that of the chalk hills of Wiltshire, Oxfordshire, and 
Buckinghamshire are fringed with numerous springs, which 
issue at their base. These springs, although thrown out 
generally by the Chalk Marl, are apparently not always at 
the top of it, but often low down in the deposit, and they 
constantly wear their point of issue from a higher to a lower 
