102 
THE MEDITERRANEAN NATURALIST 
merely resolve themselves into involuntary at- 
tempts to elude difficulties. Am ng these latter 
I would class the notion recently resuscitated 
that the sinking of the great ocean basins has 
thrown up the marginal deposits into mountain 
chains. A very slight acquaintance with geome- 
try is sufficient to show that the lateral thrust 
which, on the most favourable supposition, could 
be produced in this way is infinitesimal. Take 
the Atlantic as 2,000 miles wide and three miles 
maximum depth, what thrust could this exert on 
the shore lines, even if the bottom sunk the whole 
depth in a hundred years? The depth of the in- 
verted arch or segment is less than of the 
span, and would be represented by a deflection of 
one inch in a girder of 5*3 feet span: yet a deflection 
of this amount would probably have to be repeated 
thousands of times before the stability of tiie 
terminal supports of such a girder would be 
greatly affected. Notwithstanding this, we are 
asked to believe that the folds of the Appala- 
chians have been produced by the sinking of the 
Atlantic bed. If the whole three miles of depres- 
sion were converted into lateral thrust it would 
be insufficient. If, on the other hand, the holders 
of this theory imagine that the Atlantic bottom 
rests on a semi-fluid mass of molten matter, which 
it displaces and throws up on its margins— -an 
assumption for which there is no warrant at all, 
either physical or geological — we can only say 
that the structure of mountain chains negatives 
any such an explanation of their origin. 
Volcanic energy is, in my view, another form or 
manifestation of t. to forces which, under favoura- 
ble conditions, give rise to the expansions of the 
crust of the earth, which end in the production of 
a mountain chain; Volcanoes are surface ma- 
nifestations of this force, and any one who wishes 
to study the sr bje t from this, the first point of 
view, had better read Scrope’s classic Volcanoes 
and the interesting treatise by Professor Judd 
bearing the same title, published in the Interna- 
tional Scientific Series. 
There is, however, one aspect of the question 
which has hardly yet received the attention it 
deserves. It seems pretty conclusive that vol- 
canic energy could not have continued active 
from the dawn of geological history unless it were 
Connected with the central heat. Hopldn’s and 
Lyell’s suggestions that volcanoes may be fed 
from isolated lakes of molten rock in the earth’s 
crust does not commend itself to my mind, unless 
these lakes are themselves fed from the central 
reservoir. 
No explanation of volcanic emission which does 
not provide for getting the molten rock from a 
depth of from 20 to 30 miles is complete. In the 
first place, molten lava stands in the throat of a 
volcano in some cases 12,000 feet above the sea 
level, and with such a “hydraulic head,’’ unless it 
were fed from some deep-seated source, the co- 
lumn would lift up the earth-covering of the 
resevoir, in the same way that a man can lift 
himself by blowing down the tube of a pneumatic 
bellows. The column of rock and the column of 
lava must, to maintain stable conditions, have 
nearly the same weight; but, if these columns be 
(say) 20 miles deep, the lesser specific gravity of 
the fluid lava will account for its emission at 
heights of 12,000 feet above the sea. Unless the 
lava, fluid at the surface, is solid or nearly solid 
at its origin, it seems impossible to account for 
the phenomenon of intermittent emission at the 
surface: fdr, were the whole column of lava fluid 
and fed from a fluid reservoir, gravitation of the 
covering rock would produce continuous emission 
until exhaustion took place, instead of that inter- 
mittent emission which is the regular, or rather 
irregular, mode in which volcanic action manifests 
itself, 
Prof. Judd, in his report to the Royal Society 
on the eruption of Ivrakatoa, has shewn that when 
the mixtures of silicates of which the Krakatoa 
lavas consist contain water, then very fusible 
glasses are formed. From this fact Professor Judd 
infers that the slow percolation of water into rock 
masses from above, and the consequent formation 
of new compounds more readily acted upon by 
subterranean heat, is capable of bringing about vol- 
canic action These interesting discoveries throw 
much light upon the immediate manifestations of 
volcanic energy, but they are surface actions, and 
do not account for the pumping up of the incan- 
descent matter of the globe from below what is 
called the “crust of the globe.” Unless there were 
as I pointed out at the commencement of this 
article, a continual renewal of incandescent matter 
from a central reservoir, in the millions of ages 
