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212 | DYNAMICAL GEOLOGY. 
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The eruptions of Vesuvius are often preceded by failure or dimi- 
nution of wells and springs. But more frequent indications of an 
approaching outburst are conveyed by sympathetic movements of 
the ground. Subterranean rumblings and groanings are heard; 
slight tremors succeed, increasing in frequency and violence till they 
become distinct earthquake shocks. The vapours from the crater 
grow more abundant as the lava column in the pipe or funnel of 
the volcano ascends, forced upward and kept in perpetual agita- 
tion by the passage of elastic vapours through its mass. After a 
long previous interval of quiescence, there may be much solidified 
lava towards the top of the funnel which will restrain the ascent of 
the still molten portion underneath. A vast pressure is thus 
exercised on the sides of the cone which, if too weak to resist, will 
open in one or more rents, and the liquid lava will issne from the 
outer slope of the mountain; or the energies of the volcano will be 
directed towards clearing the obstruction in the chief throat, until, 
with tremendous explosions, and the rise of a vast cloud of dust and 
fragments, the bottom and sides of the crater are finally blown out, 
and the top of the cone disappears. The lava may now escape from 
the lowest part of the lip of the crater, while, at the same time, 
immense numbers of red-hot bombs, scoriz, and stones are shot up 
into the air. ‘The lava at first rushes down like one or more rivers of 
melted iron, but, as it cools, its rate of motion lessens. Clouds of 
steam rise from its surface, as well as from the central crater. Indeed, 
every successive paroxysmal convulsion of the mountain is marked, 
even at a distance, by the rise of huge ball-like wreaths or clouds of 
steam, mixed with dust and stones, forming a column which towers 
sometimes a couple of miles above the summit of the cone. By degrees 
these eructations diminish in frequency and intensity. The lava 
ceases to issue, the showers of stones and dust decrease, and after a 
time, which may vary from hours to days or months, even in the 
régime of the same mountain, the volcano becomes once more tranquil.’ 
In the investigation of the subject, the student will naturally 
devote attention specially to those aspects of volcanic action which 
have more particular geological interest from the permanent changes 
with which they are connected, or from the way in which they 
enable us to detect and realize conditions of volcanic energy in 
former periods. ji 
Fissures.—The convulsions which culminate in the formation of 
a volcano usually split open the terrestrial crust with a more or less 
nearly rectilinear fissure. In the subsequent progress of the mountain, 
the ground at and around the focus of action is liable to be again 
and again rent open by other fissures. ‘These tend to diverge from 
the focus; but around the vent where the rocks have been most ex- 
posed to concussion the fissures sometimes intersect each other in 
all directions. In the great eruption of Kitna, in the year 1669, a 
* A remarkably good account of the great eruption of Cot rte 
Dr. Th, Wolf will be found in Neues Jahrb, 1878, i 118, ofopaxt in Janey 
