106 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
Perhaps in addition may be mentioned earthquakes, which, 
although producing much displacement at the moment, from their 
transitory character are fractional in their value as a denuding agent 
compared with those other natural influences which, although 
quieter and less obvious in action, illustrate fully the fable of the 
hare and the tortoise. 
There is still remaining one cause that has produced very marked 
effects in shaping and directing the course of the valleys, and that 
is man; for although he acts unconsciously, we shall see, when 
speaking of the forms of the ravines, that he has played and is 
playing a very important part. 
Form of the Valleys and thew Course. 
As we approach the foot of the mountain from the plain, we first 
commence by ascending a gentle slope, covered with gardens, and 
rarely traversed by rivulets or beds of torrents, except where these 
have been artificially constructed. the great porosity of the soil sopping 
up all the mass of water brought down by the valleys. This has 
been aided by shallow wells and other irrigation-works intended to 
arrest the precious fluid. This slope continues in an unbroken 
manner till about the 200-metre contour-line, when the lower ter- 
minations of the ravines are met with. 
A valley, as a whole, may be considered, for convenience of de- 
scription, as consisting of three parts. First, its ultimate ramifica- 
tions ; secondly, the ravine proper, or ‘‘ Vallone” ; and, thirdly, its 
expanded termination or “ Lagno,” as these portions are locally 
called. 
Towards the upper part of the mountain little of the slightly co- 
herent tufas have been left; almost as soon as spread over this the 
most inclined portion by the eruption which ejected these mate- 
rials, they were swept down by the heavy rains, either during or 
soon after the eruptive period. Nevertheless, we do find on the very 
summit of the ridges, where only exposed to the amount of rain that 
fell directly upon their surface, and not to that collected from other 
areas, that tufas have been able to hold their ground. It is in such 
a way that the very interesting and instructive section of the Canale 
di Arena and others have been preserved. Such cases, however, 
are the exception, and generally the ancient lava and scoria beds 
are only covered by a thin layer of volcanic sand, held together by 
the roots of chestnut trees and broom bushes, both of which add 
much to the preservation of the mountain. 
In this region only small narrow gorges are excavated, their floor 
being formed of broken blocks of lava, and often interrupted by cas- 
cades of small depth, produced by the cutting back, as Lyell has 
called it, of old lava-flows. A variable number of these unite 
together, and roughly, where the first diminution of inclination takes 
place, form the “‘vallone” or great valley. 
The valloni, as we may conveniently call them, present certain 
general characters. As already mentioned, they commence at the 
