402 
POPULAR SCIENCE REVIEW. 
than the ordinary methods of pumping can control. It some- 
times happens that collieries are u drowned out;” but this hap- 
pens either through tapping old and abandoned workings which 
are full of water, or through following a coal-seam too near to 
the “ crop,” when this approaches the sea or a river. There can 
he no such danger with the Channel tunnel, for the works will 
be kept at least 200 feet below the sea-bed. Horizontal bore- 
holes will be kept well ahead of the faces of work ; points of 
unusual difficulty will thus.be early known, and proper precau- 
tions can be taken. 
Prof. Prestwick’s scheme differs from all those which we 
have hitherto been considering. He proposes to tunnel through 
the floor or ridge of older rocks which underlie the Secondary 
strata. The Palaeozoic rocks were bent and twisted, and sub- 
sequently very largely worn away by denudation, before the 
Secondary rocks were deposited. There is no necessary relation 
of character or sequence between the two series of rocks ; and 
no amount of investigation into the range of the Secondary rocks 
can give us any direct evidence as to the range of the Palaeozoic 
rocks which underlie them. This will be evident from an inspec- 
tion of fig. 3. Each of the sections shows that there is a 
certain regularity in the succession of % the Secondary strata, 
though all are not always present ; but, in the succession from 
the Palaeozoic to the Secondary formations, there is, in this 
area, no such regularity. In the Boulonnais the older rocks 
are immediately overlain indifferently by Gault, Lower Green- 
sand, Wealden Beds, Oxford Clay, and Great Oolite. 
While, however, we cannot by an examination of the surface 
strata form any idea as to the underlying Palaeozoic rocks, we 
are not without some knowledge as to their underground range. 
We know that these rocks come to the surface on the north- 
east of Marquise, and that they strike north-west in the direc- 
tion of Wissant and Folkestone. Along this line they will 
probably be nearer the surface than in the neighbouring dis- 
tricts. Prof. Prestwick supposes that under Folkestone they 
may perhaps be met with at a depth of from 300 to 400 feet. 
The rocks most likely to be met with are the Devonian and 
Lower Carboniferous ; they may consist of sandstones, conglo- 
merates, schistose shales, or limestones. In any one of these the 
tunnel might have to be commenced on the English side, and 
there is no certainty or probability that the rock in which the 
tunnel might begin would be continued far along the required 
line. The work would be much more expensive through these 
harder rocks than through the Chalk or clays of the Secondary 
strata ; this, although a matter of minor importance, is a point 
to be considered. Prof. Prestwich relies upon the Palaeozoic 
rocks because he believes that they would be compact and com- 
paratively free from water; the impervious clays of the Secondary 
