218 DYNAMICAL GEOLOGY. [Book IIL. _ 
rushes up from the crater. Doubtless it is the sudden escape of that 
steam which causes the explosion. 
The varying degree of liquidity or viscosity of the lava probably 
modifies the force of explosions, owing to the different degrees of re- 
sistance offered to the upward passage of the absorbed gases and 
vapours. ‘Thus explosions and accompanying scoriz are abundant at 
Vesuvius, where the lavas are comparatively viscid ; they are almost 
unknown at Kilauea, where the lava is remarkably liquid. 
In tranquil conditions of a volcano the steam, whether collecting 
into larger or smaller vesicles, works its way upward through the 
substance of the molten lava, and as the elasticity of this compressed 
vapour overcomes the pressure of the overlying lava, it escapes at the 
surface, and there the lava is thus kept in ebullition. But this 
comparatively quiet operation, which may be watched within the 
craters of many active volcanoes, does not produce clouds of fine 
dust. The collision or friction of millions of stones ascending and 
descending in the dark column above the crater, though it must 
doubtless cause much dust and sand, can give rise to but an in- — 
significant proportion of what is actually reduced to the condition of 
extreme subdivision necessary to produce widespread darkness and a 
thick far-reaching deposit of ashes. The explanation now accepted 
calls in the explosive action of steam as the immediate cause of the 
trituration. ‘The aqueous vapour by which many lavas are so largely 
impregnated must exist interstitially far down in the lava-column, 
under an enormous pressure, and at a white heat. The sudden 
ascent of lava so constituted will relieve the pressure rapidly without 
sensibly affecting the temperature of the mass, Consequently the 
white-hot steam will at length explode, and reduce the molten mass 
containing it to the finest powder, like water shot out of a gun. 
Evidently no part of the operations of a volcano has greater 
geological significance than the ejection of such enormous quantities 
of fragmentary matter. In the first place, the fall of these loose 
materials round the orifice of discharge is one main cause of the 
growth of the volcanic cone. The heavier fragments gather around 
the vent, and there too the thickest accumulation of finer dust takes 
place. Hence, though successive explosions may blow out the upper 
part of the crater-walls, and prevent the mountain from growing so 
rapidly in height, every eruption must increase the diameter of the 
cone. In the second place, as every shower of dust and sand adds 
to the height of the ground on which it falls, thick volcanic accumu- 
lations may be formed far beyond the base of the mountain. The 
voleano of Sangay, in Ecuador, for instance, has buried the country 
around it to a depth of 4000 feet under its ashes. In such loose 
deposits are entombed trees and other kinds of vegetation, together 
with the bodies of animals, as well as the works of man. In some 
cases where the layer of volcanic dust is thin, it may merely add to 
the height of the soil without sensibly interfering with the vegeta- 
' D, Forbes, Geol. Mag. vii. 320, 
