
oe: eee DYNAMICAL 
‘ 
The great floods of water which rushed down the flank lor Etna, "4 
after an eruption of the mountain in the spring of 1755, and similar 
deluges at Cotopaxi, are thus explained. 
. One further aspect of a lava-stream may be noticed here—the 
effect of time upon its surface. While all kinds of lava must, in the 
end, crumble down under the influence of atmospheric waste and, 
where other conditions permit, become coated with soil and support 
some kind of vegetation, yet extraordinary differences may be 
observed in the facility with which different lava-streams yield to 
GEOLOGY. [Boor II. 

x y | . 
this change, even on the flank of the same mountain. Every one — 
who ascends the slopes of Vesuvius remarks this fact. After a little 
practice it is not difficult there to trace the limits of certain lavas— 
even from a distance, in some cases by their verdure, in others by 
their barrenness. Five hundred years have not sufficed to clothe 
with green the still naked surface of the Catanian lava of 1381; 
while some of the lavas of the present century have long given 
footing to bushes of furze. Some of the younger lavas of Auvergne, 
which certainly flowed in times anterior to those of history, are still 
singularly bare and rugged. Yet, on the whole, where lava is 
directly exposed to the atmosphere, without receiving protection from _ 
occasional showers of volcanic ash, or where liable to be washed bare 
by heavy torrents of rain, its surface decays in a few years 
sufficiently to afford soil for stray plants in the crevices. When 
these have taken root they help to increase the disintegration; at 
last, as the rock is overspread, the traces of its volcanic origin fade 
away from its surface. Some of the Vesuvian lavas of the present 
century already support vineyards. | 
Subsidence and Elevation.—Proofs of elevation are frequent | 
among volcanic vents which, lying near the sea and containing marine | 
sediments among their older erupted materials, supply in the enclosed 
marine organisms evidence of the movement. In this way it is 
known that Etna, Vesuvius and other Mediterranean volcanoes, began 
their history as submarine vents, and that they owe their present 
dimensions not only to the accumulation of ejected materials, but also 
to some extent to an elevation of the sea-bottom. Proof of subsidence 
is less easily traced, but indications have been observed of a sinking — 
of the ground beneath a volcanic vent, as if the crust had settled 
down upon the cavity made by the discharge of so much volcanic 
material. During the recent eruption of Santorin, very decided but 
extremely local subsidence took place near the vent in the centre of 
the old crater. 
Torrents of Water and Mud.—We have seen that large 
quantities of water accompany many volcanic eruptions. In some 
cases, where ancient crater-lakes or internal reservoirs, shaken by 
repeated detonations, have been finally disrupted, the mud which has 
thereby been liberated issues at once from the mountain. Such 
“mud-lava,” (lava d’ aqua), on account of its liquidity and swiftness 
of motion, is more dreaded for destructiveness than even the true 
