281 
Allowing for any likely greater depth of a former channel, 
or for any effect the river might have had on the clay, 
Mr. Barlow considered he would be safe in allowing a 
minimum thickness of 2oft. of London Clay to exist between 
the river and the top of the tunnel—a conjecture, the 
correctness of which has been fully proved in the execu- 
tion of the tunnel; for not the slightest percolation of 
water from the river was detected during any part of the 
work. It is a question, in fact, whether a less depth might 
not have sufficed. Greater difficulties were apprehended 
in the making of the two shafts, as the gravel on either 
side of the river was known to be charged with water— 
that on the Middlesex side especially. On the north side 
of the river the ground rises at Tower Hill, and thence 
towards the City, to a height of about 4oft. above Thames 
high-water mark; and the London Clay is capped by a thick 
bed of gravel, the spring at the base of which supplies so 
many of the City pumps. This gravel, however, does not 
slope down to the river, but is cut off at the sloping 
ground. When the shaft on the north side came to 
CRAVEL © 
LNEor SPRING 
\ NDON CLAY. 
Fic. I 
be made, it was found, after passing this 14ft. of made 
ground, that the site of it was exactly on this intermediate 
hill of clay, and that the gravel which thinned out only a 
few feet higher had therefore been escaped. The shaft con- 
sequently was carried without difficulty through the 14 feet 
of made ground and 44 feet of London Clay toa depth of 58 
feet. On the south side, the gravel, alluvial, and made soils, 
a, were found to be 35 feet thick, and charged with water, 
which rose and fell with the tide (highest just before low 
water) in the river, to the extent of 3 feet. By the use of 
iron-tubing, a passage through has been effected, although 
with more difficulty, and the shaft carried to a depth of 
17 feet in the clay 4, or a total depth of 52 feet. Between 
these two points Mr. Barlow laid the tunnel at a slight 
curve, so as to have a depth in the centre 18 feet greater 
than at the ends. The shafts have a diameter of 10 feet 
at top and 8} feet at bottom. The tunnel is 7 feet in 
diameter, and is formed by cast-iron tubing in lengths of 
1} feet each, each ring being composed of thin segments 
with a key piece. An iron shield, devised by Mr. Barlow, 
was pushed on in advance of the work, and the tubes 
fixed in as soon as the requisite length of excavation had 
been completed, and the small space left between the 
clay and tube filled in at once by concrete. The work 
was carried on day and night, and advanced without 
interruption. No subsidence occurred in any part, and a 
regular and steady progress of 9 feet daily was made. 
Mr. Barlow, junior, is the engineer of the work, 
and Mr. Greathead the contractor; and the whole 
plan and execution of the work does very great credit 
both to the projector, Mr. Barlow, and to the en- 
gineer and contractor. With the exception of a slight 
leak in the iron tubing of the shaft on the south side, and 
which has been remedied after but a short delay, not the 
slightest mishap has occurred in the execution of the 
work, nor has a single fatal or even serious accident 
happened to any of the men. Last month the junction 
between the north and south side was effected, and the error 
of direction from the two ends was found not to amount 
to one inch. The passage under the river will be made in 
an omnibus, by means, probably, of a stationary engine ; 
and lifts on either side will take the passengers up and 
down. A few minutes will suffice for the journey, and it 
is hoped that the work may be opened to the public ina 
few weeks. A remarkable feature of this interesting work 
has been its small cost. Mr. Barlow’s estimate was 
16,000/., and it is now tolerably certain that the entire cost 
will be under 18,0007. Another feature has been the 
rapidity of execution. The shaft on the north side was 
commenced on the 16th of February last. On the 
26th of April the tunnel, which is 1,320 feet long, was 
commenced, and on the 8th of October the passage under 
the river was safely effected. Before the public at large 
was aware that another Thames Tunnel was completed, 
the old London wonder has been duplicated. 
As the object has been chiefly to speak of the geological 
problem, a section is annexed to show the structure of the 
ground at this part of the river. 
| 
ees 
[| SHAFY 
| 
Very few fossils were found, and these chiefly in the 
clay of the north shaft. They consist of Cryptodon 
angulatum, Corbula globosa, Pinna affints, Dentalum 
nitens, Fusus, and column of Pentencrinite. In the 
tunnel pieces of wood pierced by the 7evedo, and some 
teeth of Shark, were met with. As the London Clay 
thickens to the west and north-west, and extends uninter- 
ruptedly to Windsor on the one hand and to Watford on 
the other, covered in places near London by beds of gravel 
and sand, which never, however, exceed thirty to forty 
feet in thickness, and rarely exceed fifteen feet, there is 
thus in this area a formation which lends itself singularly 
well to the construction of subways and tunnels. 
J. PRESTWICH 
THE MAMMALIA OF SWITZERLAND. 
fraune des Vertébrés de la Suisse. Par Victor Fatio 
Dr. Phil. Vol. I. Azstotve Naturelle des Mammi- 
Jéres. (H. Georg, Genéve et Bale, 1869.) 
] FATIO’S book is one which will be valuable to all 
* Swiss naturalists, and to those who take an interest 
in the productions of the mountainous region of central 
Europe. It fulfils its function admirably as far as it goes, 
but, like all works treating of local Faunas or Floras, its 
general interest is diminished in proportion to the diminu- 
tion of the area investigated. It has not, for example, the 
importance or value to a distant reader which such a 
volume as the “ Naturgesch. der Saugethiere Deutschlands 
und Mitteleuropas” of Blasius possesses ; but to local natu- 
