GEOLOGY OF THE MONT CENIS TUNNEL. 
351 
whether made in mines or borings for water, but in no case to 
half the depth of the central part of the tunnel, shows the rise 
to be one degree in about 54 feet as an average and the stratum 
of invariable temperature to be about 60 feet. This is the rate 
not only when the boring commences near the sea but also 
near Geneva in a deep sinking commenced 1,600 feet above 
the sea. The calculated temperature, therefore, in the tunnel 
under the crest, would certainly be enormously higher than the 
observed rate. It may perhaps be suggested that something 
should be allowed for the steep slope of the mountain, but this 
slope is not sufficient to make the distance from any point of 
the surface much less than the distance from the crest. 
It may be considered, then, that in this matter the result of the 
observation of temperature of the rock within the tunnel tends 
to shake confidence in the conclusions hitherto received with 
regard to the average rate of increase of heat in the interior of 
the earth. And this is the case in more ways than one. It is 
hardly possible to imagine a case more strictly average in its 
general nature than the one before us. The rocks in which 
the observations have been made are absolutely the same, 
geologically and otherwise, from the entrance to the tunnel on 
the Italian side for a distance of nearly ten thousand yards. 
They are not faulted to any extent, though highly inclined, 
contorted, and subjected to slight slips and slides. They con- 
tain little water and no mineral veins. They consist, to a very 
large extent indeed, of silica, either as quartz or in the form of 
silicates chiefly of alumina, and the small quantity of lime 
they contain is a crystalline carbonate. Such rocks appear to 
possess no cause that could affect a fair estimate of the rate of 
increase of temperature. The result, if accepted, would reduce 
the rate of increment to a degree in about a hundred feet. 
It is very much to be regretted that we have not access to 
all the observations made, and that the character of the observa- 
tions is not so good as the subject demands. At the same time 
there is no doubt that the general conclusion is correct, and 
that the average just stated is a near approximation to a 
correct one. It is certain that in this tunnel, which affords 
the best opportunities for close and accurate observation ever 
obtained, the result is such as to demand a reconsideration of 
all geological arguments based on the rapid and regular in- 
crease of temperature towards the centre of the earth. The 
temperature of the water already quoted at two points would 
seem to correspond sufficiently with the observed temperatures 
of the rock. Thus, in the somewhat powerful spring tapped 
in 1861 when the work had only proceeded a comparatively 
short distance, the temperature was 64°, and the superincum- 
bent rock was about 2,500 feet. The rate is also about one 
