OF MONTE SOMMA AND VESUVIUS. 49 
had been at work. It is possible that this breccia was deposited 
whilst the volcano was still in a feeble state of activity. 
Both of these latter-mentioned facts are but accessory evidence 
in favour of a long period of apparent extinction which is shown 
chiefly by the change of character of the ejectamenta. 
That this change was not in the primitive magma, but rather 
produced by the varied influences brought to bear upon it in its 
journey from its source to the surface, will be demonstrated as we 
proceed in our study. 
Probably the old vent was plugged up by a mass of leucitic lava, 
a remnant of the last of the permanent prehistoric eruptions, which, 
from flagging of the primary volcanic forces or their concentration 
at other foci, had cooled there. 
In the extinction of a volcano a series of counteracting influences 
are at work. As the result of former eruptions, or from other 
causes, the pressure of the igneous matter at its origin is diminished, 
and the remnants of the fluid or plastic mass occupying the chimney 
will gradually cool by solfataric or fumarolic action. In this case 
the consolidation will proceed from the surface downwards, or 
irregularly, if the heat be carried away by thermal waters. The 
magma still deeper will gradually assimilate water, resulting in its 
tension rising to a certain point at the expense of its temperature. 
If this be sufficient to overcome the superincumbent obstacles a 
violent explosion or paroxysmal eruption will occur. On the con- 
trary, if the temperature is low in the first case, or subsequently 
descends sufliciently for the mass to solidify, an eruption will be 
prevented, and the solidified mass will be an obstacle to any further 
attempts at a rupture. 
lf at a future time igneous matter tries to force its way upwards 
in the same neighbourhood, it will preferably be by the side of 
the old axis, which itself would offer the greatest resistance. It is 
probably in this manner that a variation of the eruptive axis, as in 
Monte Somma and Vesuvius, is brought about. 
Era of Paroxysmal actwity. 
The leucitic lavas and their derivatives are overlain by a series of 
pumiceous tufas which form a kind of mantle to the ancient cone of 
Monte Somma. Breislak* was the first to describe this superposi- 
tion. He was followed by L. von Buch tf, who fell into the error of 
classing the pumice beds of Somma and those of the district of 
Naples together. This is easily seen not to be the case for the fol- 
lowing reason. ‘The tufa consists of pumice in which various 
trachytic fragments are common, but amongst which neither the 
leucitic lava nor large series of ejected blocks like those of Monte 
Somma, are to be encountered. On the other hand, the trachyte 
does not occur at Monte Somma; neither do the pumices resemble 
each other in composition. 
* ‘Voyages physiques et lithologiques dans la ‘Cte 1801. 
+ ‘Description physique des iles Canaries, p. 341 (1836). Traduit par 
C. Boulanger. 
Oo 35G. S.No, 157. B 
