337 
THE GEOLOGY OF THE MONT CENIS TUNNEL. 
Br Professor 1). T. ANSTEI), M.A., F.B.S., For. Sec. Geol. Soc. 
With a Geological Map and Section. 
[PLATE LXIV.] 
T HE approaching completion of the great tunnel through 
the Alps, the most interesting and remarkable engineer- 
ing work — certainly the most daring and speculative work — - 
of any that has yet been undertaken for railway purposes, 
renders an account of its progress and of the condition of the 
rocks pierced especially interesting. At the present time, 
more than half the total distance has been completed from the 
Italian and nearly half on the French, and the rock at both 
ends being now the same, the structure of the whole mass of 
mountain penetrated may be estimated with something ap- 
proaching to certainty. A recent visit by the writer of this 
article has induced him to believe that an account of the 
geology of the tunnel rocks would be found to be of general 
interest. His object will be not so much to describe the*, 
numerous and ingenious inventions and mechanical appliances 
adopted for perforating the rock and ensuring ventilation for- 
the great distance of nearly four miles from each end, as to 
point out the condition and nature of the rocks pierced in 
comparison with those seen on the surface. We thus get an 
insight into the real structure of the Alps at this point, illus- 
trated by a perfect section between seven and eight miles long, 
obtained by the aid of a tunnel through highly metamorphic 
rock, the central part being more than 5,000 feet perpen- 
dicularly below the actual surface of the ground. Besides the 
geological structure, several important physical problems will 
be found to be involved and will receive some explanation. 
The part of the Alpine mountain system through which this 
tunnel is being pierced, is the watershed that separates the 
waters of the Dora, one of the principal tributaries of the 
Upper Po, entering the Po at Turin, and those of the Arc, a 
VOL. ix. — NO. xxxvii. z 
