THE MEDITERRANEAN NATURALIST 
16 $ 
that have elapsed since the earth became habitable 
volcanic energy would have died out, whereas, 
according to the best geologists, it is potentially as 
active now as ever. 
It is a very remarkable fact, that frequently in 
grea' ; ranges of mountains the crystalline nucleus 
has been penetrated by igneous dykes long after 
the elevation of the range, and volcanic cones have 
been built upon the highest points. This holds 
good with the Caucasus, where volcanic eruptions, 
which took place at the close of the Tertiary epoch 
acccording to E. Favre, only exercised a local effect 
on the upheaval of the chain. The most conside- 
rable cone and the highest peak is that of Fibrous, 
which has arisen in the midst of crystalline rocks 
w 7 here the eastern part of the Caucasus reaches its 
greatest width. We may not unnaturally ask 
ourselves why volcanic action should manifest 
itself specially in this way, rather than that the 
lava should break out at lower levels? It would 
seem to point to the persistence of a focus, or of 
foci of heat, under the range, remnants of the 
heat energies which originally, by expansion of 
the heavy overlying sediments which sealed them 
up, gave rise to the range. 
Yolcanic action has, in one form or another, al- 
ways been present during the building of moun- 
tain ranges. The sedimentary beds are often inter- 
stratified with lava flows of great thickness, and 
seemingly the combined sediments and lava sheets 
have had to be piled up to a great thickness before 
the energies necessary for mountain building have 
accumulated sufficiently to initiate mountain move- 
ments. That this has been the case we see a 
good example in our own Snowdon, where the 
sheets of trappean rock partake of the folds of the 
mountain equally with the sedimentary beds. 
From this it would appear that where there has been 
great surface emission there has not been contem- 
porary mountain building — except in a plateau- 
like form, as in the Deccan in India and Colorado 
in North America. If, as I infer, mountain build- 
ing and volcanic action are different results of the 
same heat energies, it follows that where there is 
easy surface overflow there cannot be that intense 
folding and crushing which are the predominant 
characteristics of a great mountain range. 
Our direct knowledge of volcanic action is li- 
mited to surface phenomena. Mechanical know- 
ledge almost as certainly leads to the inference 
that it is really deep-seated. We cannot say 
what takes place in the laboratory of nature 30 
miles below our feet, but that variations and long- 
pulsations of temperature take place we must 
reasonably suppose. We have seen that the piling 
up of sediments is one cause of this, but doubt- 
less there are others we can only dimly guess at. 
Emission at the surface of molten lava will produce 
a movement of fresh magma towards the base of 
the column or pipe: this means a renewal or 
accession of heat, and it also means further che- 
mical reaction and melting of surrounding rocks. 
Expansion in volume of this magma, be it ever so 
minute, will show in the volcanic column like the 
mercury in a thermometer. Doubtless there are 
resevoirs of lava in the solid crust itself fed from 
the. nucleus, and an alteration of volume in the 
solid surrounding rock, such as accompanies a 
small change of temperature, will affect the lava 
column in the same way as an alteration of the 
bulk of the molten matter of the reservoir itself. 
The late Mr. Mallet considered volcanic action 
to be due to the crushing in of the crust of the 
earth following upon secular refrigeration, and his 
theory was therefore on the same basis at the 
“contraction” theory of mountain formation. If, 
however, this theory is incapable of explaining 
mountain upheaval, still less can it account for 
volcanic action; for, as is shown by the investi- 
gations of the depth of the “level-of-no-strain” 
crushing ceases a few miles below the surface, 
Yolcanic action, as already shown, is initially deep 
seated, and volcanoes must be fed from a zone of 
the earth at depths so profound as to be well 
within the contracting magma. Now, as con- 
tracting matter cannot force itself up to the surface, 
we must look to other agencies for the pushing 
up of the incandescent matter of the interior, 
which only becomes molten by the relief of pres- 
sure on nearing the surface. This force is to be 
found, I believe, in variations of temperature, 
which increase the local bulk of particular sections 
of the earth, 
The problems discussed in this series of articles 
are of a very difficult nature. They demand a 
great variety of knowledge on the part of him who 
would investigate them. 
