GEOLOGY OF TIIE MONT CENIS TUNNEL. 
347 
and some much larger. The beds, being inclined at a high 
angle, are easily removed in large masses. 
All geologists familiar with the appearance of rocks in mines 
and tunnels are aware of the variety of dip that frequently 
shows itself in the interior of the earth, and of the fact that 
the dip of the rocks at the surface is by no means always coin- 
cident with that of the interior. The difference may be small, 
but it is generally clear and well marked. It may often be 
observed even in quarries and deep railway cuttings. In the 
Mont Cenis tunnel perhaps the most striking fact seems to be 
that of a constant variety of dip, but always within certain 
limits. This is no doubt due in a very great degree, if not 
entirely, to the contortion consequent on squeezing ; and this 
again is the result of the elevatory forces that have brought up 
these materials, originally deposited horizontally on a sea 
bottom, and now the crests of great mountain chains many 
thousand feet above the sea. With the variety of dip is observed 
at the same time a singular complication of small slips and 
troubles. None of these seems to have been important or due 
to any other cause than the same contortions so prominently 
illustrated. Still the phenomena of glazed surfaces, slicken- 
sides, and smoothed, polished faces in close contact are to be 
observed almost incessantly, and they are accompanied by a few, 
but very few, open cracks and fissures, generally consisting of 
small pockets partly filled or lined with crystals of quartz, calcite, 
or dolomite, and crystals of iron and copper pyrites. Galena 
and blende are also found but are very rarely distributed, and 
in exceedingly small quantities, in the same localities. 
So far, then, as the general appearance and construction 
of the rocks can be determined by aid of the extensive and 
remarkable excavation now in progress and nearly completed, 
we may say that they confirm what has been already observed 
in mines and tunnels of smaller extent in similar rock, and show 
the phenomena of metamorphosis of secondary rocks (origin- 
ally, no doubt, fossiliferous and regularly bedded) to present the . 
same general character in the interior of the earth that they 
do nearer the surface. No intimation whatever of the exist- 
ence of older or harder or more altered rock is seen. The rocks 
are not, perhaps, quite so hard or tough in the middle of the 
tunnel as they were found to be near the entrance, but they 
retain all their characteristics throughout, and we may fairly 
presume that the mass of the metamorphosed jurassic rock not 
yet converted into granite or protogine retains now, both in 
the interior and near the surface, the same condition that was 
induced when the metamorphism was first effected, and the 
elevation of the mountain mass of the Alps was completed. All 
the phenomena of slides and small faults conform accurately 
