THE CHANNEL TUNNEL. 
397 
at Calais, that these lower strata are there absent, and that the 
Gault (with a thin representative of the Lower Greensand be- 
neath it) there rests upon the Palseozoic rocks. 
The vertical sections (figs. 1 and 2 of Plate CXIY.) show the 
complete successions of strata near each shore, and some attempt 
has here been made to indicate the characters of the various 
beds. Horizontal shading indicates clay or shale, and con- 
sequently impervious beds ; dots indicate sand, or pervious 
beds ; vertical lines indicate limestones or calcareous beds, which 
are not usually very pervious in the mass of the beds, but 
only by reason of the joints or fissures which traverse them. 
The greatest thickness of the ‘various beds is also shown in figs. 
1 and 2 ; it often happens that the thickness of a bed varies 
in different places ; this is shown in the longitudinal sections 
(figs. 3 to 6). The continuity of a bed on the surface is shown 
in the map ; the continuity below ground, which is of more 
importance, is shown in the sections. 
The map and sections, therefore, contain, as well as can be 
expressed on so small a scale, all the more important geological 
data which we at present possess for testing any proposed line 
of tunnel. But it may be necessary to state that the informa- 
tion here given is of very unequal value. The outcrops on the 
surface are sufficiently exact ; the lines of the Chalk, Gault, 
and Lower Greensand beneath the Channel are probably tole- 
rably near the truth ; but the Wealden and Oolitic lines beneath 
the Channel are wholly conjectural. The evidence upon these 
points is excessively meagre, and future researches may show that 
the actual outcrops between Dungeness and C. Grisnez are very 
different from those here sketched. With the sections the case 
is different, and I have little doubt that the general geological 
structure along the various lines is very much as is there shown. 
We will now examine the various strata in respect to their 
suitability for tunnelling. Applying the first test, that of im- 
permeability, we at once discard the Lower Greensand, the 
Hastings Beds, and the Portland Beds.* These will certainly 
contain water in considerable quantity; most of the minor 
divisions of each of these formations are highly porous, and the 
divisions which are less so are not thick or continuous. No 
* To describe in detail all the characters of the various divisions would 
carry this article to too great a length. Mr. F. Drew has described the 
English beds in the “ Memoirs of the Geological Survey/’ Sheet 4 ; the 
French strata have been studied by Itozet, Hebert, Rigaux and others. The 
whole question has been stated by Prof. Prestwich in his paper already 
quoted. The (i Geology of the Straits of Dover ” (with especial reference 
to the Channel tunnels) was described by myself in the “ Quarterly Journal 
of Science,” April 1872. 
