OF MONTE SOMMA AND VESUVIUS. ; 105 
say, the pumice floating along on the top of the torrent. If we 
think over the effect of such a stream, we may well comprehend its 
transporting power; for although the pumice is lighter than water, 
the impulse with which it strikes and presses upon obstructing 
objects, is far greater than that of the fluid which supports it, which 
divides and finds its way around the obstruction. The effects are 
analogous to the destructive action of floating bodies, such as ice, on 
the piers of bridges. The wearing action of this floating pumice is 
such that where a fine-grained lava-stream forms the floor of a valley, 
its substance is eroded into peculiarly-shaped cavities with rounded 
sides, so polished as to appear like the result of glacial action, or 
even the handiwork of a lapidary. 
Little undergrowth or grass exists on the middle and upper parts 
of the mountain, so that aqueous action can have full play. To 
this must be added other denuding influences, and amongst them 
we will first discuss that of ice. 
In the months of December, January, February, and March, frost 
is rarely absent during the night, which arises from the northern 
aspect of most of the great valleys. Besides this, the great amount 
of evaporation brought about as the result of the porosity of the 
rocks, the currents of air sweeping along the valley, and the 
little sunshine that enters them, combine to produce a very low 
temperature. 
The ice, by its expansion as it forms, splits the lavas and scorie, 
and separates large crusts from the damp tufa escarpments, exposing 
fresh surfaces, to be, in their turn, submitted to the same action. 
Not only does it destroy lava-currents, by separating them in blocks, 
but those especially containing large leucite crystals are molecu- 
larly broken up, the water finding its way along the fractures of 
this mineral, causing it to expand and, as it were, explode the rock. 
If we break up a coarse, decomposed, rather vesicular leucitic lava, 
we may often see a crystal that has burst its way into one of the 
cavities. This, of course, is aided by the kaolinization of the 
mineral; but that decomposition of leucite seems to be greatly 
facilitated by expanding ice and water caused by the continual 
thawing and refreezing. 
During the great heats of summer another agency similar to 
ice-action is at work, ‘The porous rocks, loaded with moisture by 
capillary attraction from below, suddenly receive a hot summer sun, 
which, in a short time converts the superficial water in the saturated 
tufa into vapour, and by its expansion a constant crumbling is 
produced in the less coherent beds. 
Last, but not least, we must take into the calculation the effects 
of wind. Any one who has had the misfortune to be caught in one 
of the ravines during a storm, especially where there are high escarp- 
ments of loose materials, besides having his face and hands cut by 
rock particles carried by the violent gusts that sweep the valley, 
will have his courage fully tried by large masses that are detached 
and rolled down, often making a veritable avalanche of stones, from 
which he can only dodge but not retreat. 
