GEOLOGY OF TIIE MONT CENIS TUNNEL. 
343 
—country, rising in Mont Pelvoux to the height of more than 
1 3,000 feet, though usually much lower. This district is more 
remarkable for the narrowness and ruggedness of its nume- 
rous valleys than for its lofty peaks, although these include 
Monte Viso and Mont Grenevre as well as Mont Pelvoux. It 
has also some passes, but they are inconvenient. The valleys 
are not only inconvenient and inaccessible, but offer no 
advantages to draw away the traffic from the direct line in 
which it has long been carried. For the purposes of a railroad 
reasonably accessible, valleys and narrow crests are far more 
important than low mountains ; and so far as a tunnel is con- 
cerned, it matters little whether it is carried under a pass or 
under a mountain top. Thus it is that the valleys of the Isere 
and its tributaries on the Savoy side, and that of the Dora on 
the Italian side, are those which the tunnel can most usefully join. 
The geological interest of a tunnel thus driven through a 
mountain axis — the tunnel being not only by very far the 
longest but the deepest below the surface that has ever been 
attempted — is very great and very varied. We are enabled by 
it, first, to compare the rocks as they exist within the earth 
under great pressure with the same rocks cropping out on the 
surface. Secondly, we are in a position to judge of the con- 
dition of rocks in the interior of the earth with regard to 
small slips, slides, faults, cavities, and various phenomena of 
contortion, as well as slight disturbance on a very large scale, 
and to an extent elsewhere unknown. Thirdly, we learn the 
condition of the interior with regard to water. Fourthly, we 
obtain direct knowledge of the temperature of the interior of 
the earth under circumstances of great interest. 
All these matters are now determined in the Mont Cenis 
tunnel so far as they are likely to be determined, for the works 
are carried on nearly half a mile beyond midway from the 
Italian side ; they are already under the culminating point of 
the crest, and the rocks now being removed from the French 
side are precisely identical with those long excavated from the 
other end, proving that there is nothing more to be learnt in 
this respect. 
The rocks penetrated in the Mont Cenis tunnel consist, as 
we have seen, of highly metamorphosed materia], and no fossils 
have yet been discovered in them; nothing organic, indeed, 
has been in any way indicated beyond the existence of anthra- 
cite, of which there are beds of some thickness and importance. 
All the rocks are described as belonging to the jurassic series, 
but under that name must be included the lias, and possibly 
some of the rocks of the recently established Khsetic series 
below. Similar beds of the same series contain fossils in other 
parts of Savoy not very distant, and of the general fact that the 
