586 
ELEMENTS OF GEOLOGY. 
M. Fournet, in his description of the metalliferons gneiss 
near Clermont, in Auvergne, states that all the minute fis¬ 
sures of the rock are quite saturated with free carbonic acid 
gas; which gas rises plentifully from the soil there and in 
many parts of the surrounding country. The various ele¬ 
ments of the gneiss, with the exception of the quartz, are all 
softened; and new combinations of the acid with lime, iron, 
and manganese are continually in progress.* 
The power of subterranean gases is well illustrated by the 
stufas of St. Calogero in the Lipari Islands, where the hori¬ 
zontal strata of tuffs, forming cliffs 200 feet high, have been 
discolored in places by the jets of steam often above the boil¬ 
ing point, called ‘‘ stufas,” issuing from the fissures; and 
similar instances are recorded by M. Virlet of corrosion of 
rocks near Corinth, and by Dr. Daubeny of decomposition 
of trachytic rocks by sulphureted hydrogen and muriatic 
acid gases in the Solfatara, near Naples. In all these in¬ 
stances it is clear that the gaseous fluids must have made 
their way through vast thicknesses of porous or fissured 
rocks, and their modifying influence may spread through the 
crust for thousands of yards in thickness. 
It has been urged as an argument against the metamor- 
phic theory, that rocks have a small power of conducting 
heat, and it is true that when dry, and in the air, they differ 
remarkably from metals in this respect. The syenite of Nor¬ 
way, as we have seen, p. 558, has sometimes altered fossilifer- 
ous strata both in the direction of their dip and strike for a 
distance of a quarter of a mile, but the theory of gneiss and 
mica-schist above proposed requires us to imagine that the 
same influence has extended through strata miles in thickness. 
Professor Bischof has shown what changes may be superin¬ 
duced, on black marble and other rocks, by the steam of a 
hot spring having a temperature of no more than 133° to 
167° Fahr., and we are becoming more and more acquainted 
with the prominent part which water is playing in distribu¬ 
ting the heat of the interior through mountain masses of in¬ 
cumbent strata, and of introducing into them various miner¬ 
al elements in a fluid or gaseous state. Such facts may in¬ 
duce us to consider whether many granites and otheip rocks 
of that class may not sometimes represent merely the ex¬ 
treme of a similar slow metamorphism. But, on the other 
hand, the heat of lava in a volcanic crater when it is white and 
glowing like the sun must convince us that the temperature 
of a column of such a fluid at the depth of many miles ex¬ 
ceeds any heat which can ever be witnessed at the surface. 
* See Principles^ Index^ “ Carbonated Springs,” etc. 
