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POPULAR SCIENCE REVIEW. 
condition of pure gold and silver veins in quartz rock favour 
suck an idea ? 
Again_, be it tke absolute combustion of sodium^ potassium_, 
silicon_, and magnesium that may be assumed as tke cause of 
volcanic action_, or be it tke keat generated in some slow 
modified combustion effected by tke decomposition of pre- 
viously existing compounds and tke re- composition of new ones, 
it still seems to me tkat tke very mechanical action of tke over- 
flow of lava and tke ejection of gases goes far to skow tkat it 
is chemical action of some sort tkat gives rise to tkose me- 
chanical effects. 
Fig. 1 . — Ideal Section of a Volcano. 
S R, Solid Stratified and other Rocks ; M L, Source of Molten Lava ; H, Solid Hypogene 
Rocks j V' V, Crater formed of Lava-streams (e e) and layers of ashes {z z). 
If, instead of steam, as in tke Geysers, gases were accumu- 
lated in pockets of tke solid eartk-roof (o), above tke molten 
lava-source (m l) in tke deptks of tke eartk, or in kuge bladders 
(o), or even disseminated in innumerable mere bubbles in tke 
substance of tke lava-source itself (as at x), tke continued in- * 
creasing compression of suck gases, by tkeir mere accumu- 
lation witkin a limited space, no matter kow large, would in 
tke end give rise to sufficient meckanical force to cause a 
general flow of tke wkole lava-mass towards any volcanic 
ckimney or fissure wkere tke pent-up gases migkt 
liberate tkemselves and escape, or wkere tke kot lava, forced 
up, as in Etna, to 11,000 feet, migkt overrun tke summit (a), 
or, by its weight and downward pressure (<x->c) burst open 
a new crater (d) tkrougk the incoherent sides of the cone (v' v), 
as in tke last eruption which broke out far below tke central 
orifice, near tke base of tke monticule Frumenta (Fig. 2, p). 
Tke gases liberated — whether by easy escape or by violent 
explosion — the pressure upon the lava-source would be re- 
