GEOLOGY OF THE MONT CENIS TUNNEL. 
345 
seen at the surface as they crop out, may be compared with 
the same rocks under a cover and pressure of about 3,500 feet 
of similar rock. By proceeding a few miles to the east, and 
mounting the Mont Frejus to the observatory erected for the 
use of the tunnel, a height of 9,676 feet above the sea is 
attained, being about 5,463 feet above the tunnel below. The 
middle point of the tunnel is thus nearly 20,000 feet from the 
earth’s surface horizontally, and between 5,000 and 6,000 feet 
vertically below ; and we are enabled with facility to compare 
rocks of the same series altered by metamorphosis in the same 
manner, but in which the effect of exposure to diminished 
pressure and increased weathering is so strongly marked. 
It may be sufficient to describe the tunnel rocks as follows : 
— Entering on the north side from Modana, after penetrating 
for a short distance the talus of fallen and weathered rocks from 
the mountain side, the cutting is carried through more than 
2,000 metres, or nearly a mile and a quarter, of coarse grits, fine 
grits, quartzites, quartz conglomerates, schists of various kinds, 
chiefly of a pale-grey colour, some calcareous rocks, and bands 
of some thickness of imperfect anthracite. Crystals of blende 
and of galena have been found in grits, but no veins or strings 
of ore. Crystals of dolomite were found occasionally. The 
principal bands of anthracite were about 1,200 yards from the 
entrance, and very marked bands of quartzite were met with 
at about 2,500 yards. Many of the rocks in this part of the 
work were tough, and required much time and patience to bore 
through and blast. They were much harder than those at the 
other end, and rendered the progress much slower. The rocks 
of this group were found to be a good deal contorted, and, 
though on the whole dipping towards the northern valley at a 
very high angle, they appear to have shown marks of great 
squeezing, affecting the different rocks very differently. No 
important faults were observed. 
The rocks of the last-named group terminate with quartzite, 
and were succeeded by a large series of limestones and gypsums. 
Proceeding up the valley to cross the mountain crest to Bar- 
donneche, the rocks that crop out successively do not give a very 
clear indication of the series met with inside the tunnel until 
we reach the quartzite, which is precisely identical. The 
schists and psammites are altered in colour, texture, and hard- 
ness, but they remain evidently of the same nature. The slight 
disturbances, and especially the marks of extreme squeezing, are 
not observable in the surface rock. 
The gypsum deposit next succeeding continues for about 
1,650 yards, and consists of a series of limestones and clays, 
containing gypsum in very large quantities in an amorphous 
state. On the surface these gypsums are presented in large 
