108 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
stream, this, not widening as it is cut back, gives to the valley, or 
that portion cut through the lava, a very narrow form with perpen- 
dicular or overhanging sides, of which we have fine examples in the 
Valloni Sacramento, Cancherone, and Sanseverino. 
We therefore see that the upper portion of the sides of a vallone 
are broad and inclined, whereas the lower are narrow and per- 
pendicular, often reaching such depths as 120 metres or more, as 
towards the upper part of the Vallone Breislak. This particular 
type of gorge or valley has been compared to a boat; but perhaps 
it would be better represented by the letter Y. 
It is usually this middle division of the valley that yields the finest 
sections of the later ejections of the volcano. 
The third portion, or lagno, as it is locally called, is generally the 
result of the union of two or more valloni. The character of this 
inferior or ultimate division of the ravines also is dependent for its 
special characteristic upon the further diminution in the inclination 
of the mountain-slope, which rarely reaches 15° and is usually nearer 
10°. These lagni may be described as broad flat-bottomed valleys 
bounded by nearly, if not quite, perpendicular walls, which are high 
at the junction of the lagno and vallone, and gradually decrease 
until they are reduced to mere low banks a metre or so in height. 
As a consequence of this it will be seen that the bed of the lagno is 
much nearer the horizontal than the mountain’s side through which 
the water has cut its way. Itis rather uncommon to find an old 
lava-flow forming the bed of the lagno, and still rarer to find any 
such cut through. 
The torrent that has been formed by the union of the water falling 
in the basin of the first division of the valley, descends into the 
second part carrying but a small load of materials. Here, however, 
it cuts away the incoherent tufa where this is within its reach, 
or is added to by the water and disintegrated substances from the 
sides of the vallone. Thus laden it dashes over the precipices, 
breaking up the lava-beds, detaching and carrying forward the sub- 
jacent scoria and other substances. The cascades act as so many 
weirs absorbing a considerable amount of the downward impulse of 
the stream, so that when the lagno is reached, where further retar- 
dation takes place from diminished fall, the larger of the transported 
materials begin to be deposited, and this continues until the water 
reaches beyond its self-cut banks. Then, if left alone, it breaks up 
into a number of little rivulets which gradually lose themselves by 
sinkage, and all the finer alluvial materials are left on the surface. 
It is just this peculiarity that renders the lower slopes of the moun- 
tain one of the richest gardens in Europe. This form of aqueous 
denudation, aided by the other agencies already mentioned, produces 
certain definite results. First there is the removal of nearly all the 
softer materials of the upper portion of the mountain, which takes 
place from above downwards. Next we have the formation, widening, 
and deepening of the middle division of the valley or the valione 
proper, and lastly a deposition of the transported material around the 
toe of the mountain. As a consequence of these operations a kind of 
