262 - -DYNAMICAL GEOLOGY. — -[Boox IIL. 
the Miocene upheaval of the Alps and other middle Tertiary 
movements in Europe. 
But in the ordinary phase of volcanic action,.marked by the 
copious evolution of steam and the abundant production of dust, slags 
and cinders, from one or more local vents, it is manifest that one 
main cause of volcanic excitement is the expansive force exerted by — 
vapours present in the molten magma from which lavas proceed.’ 
Whether and to what extent these vapours are parts of the aboriginal 
constitution of the earth’s interior, or are derived by descent from 
the surface, is still an unsolved problem. So large a proporticn 
being steam, much of the superheated vapours of volcanic vents 
may have been supplied by the descent of water from above ground. 
The floor of the sea and the beds of rivers and lakes are all leaky. 
Rain sinking beneath the surface of the land, percolates down cracks 
and joints, and infiltrates through the very pores of the rocks. 
The presence of nitrogen among the gaseous discharges of volcanoes 
indicates no doubt the decomposition of water containing atmospheric 
gases. The abundant volcanic sublimations of chlorides are such as 
might probably result from the decomposition of sea water. 
Accordingly, there has arisen a prevalent belief among geolo- 
gists, that it is to the enormous expansive force of perhaps 
white-hot water imprisoned in the molten magma at the roots of 
voleanoes that the explosions of a crater and the subsequent rise of a 
lava-column are due. It has been supposed that, somewhat like the 
reservoirs in which hot water and steam accumulate under geysers, 
reservoirs of molten rock receive a constant influx of water from the 
surface, which cannot escape by other channels, but is absorbed by 
the internal magma at an enormously high temperature and under 
vast pressure. In the course of time the materials fillmg up the 
chimney are unable to withstand the upward expansion of this im- 
prisoned vapour and water, so that, after some premonitory rumblings, 
the whole opposing mass is blown out, and the vapour escapes in the 
well-known masses of cloud. Meanwhile, the removal of the over- 
lying column relieves the pressure on the Java underneath, saturated 
with vapours or superheated water. This lava therefore begins to 
rise in the funnel until it forces its way through some weak part of the 
cone, or pours oyer the top of the crater. After a time, the vapour 
being expended, the energy of the volcano ceases, and there comes a 
variable period of 1epcse, until a renewal of the same phenomena 
brings on another eruption. By such successive paroxysms it is 
supposed that the forms of the internal reservoirs and tunnels are 
changed; new spaces for the accumulation of superheated water are 
opened, whence in time fresh volcanic yents issue, while the old ones 
gradually die out. 
An obvious objection to this explanation is the difficulty of 
conceiving that water should descend at all against the expansive 
' See Reyer’s Beitrag zur Physik der Eruptionen, Vienna, 1877, where the part taken 
by absorbed gases and vapours is cogently advocatcd. 
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