Dr. H. J. Johnston-Lavis—On the form of Vesuvius, etc. 447 
the south side than on the north, and doubts what compensation the 
modern lavas, etc., have given. On the north side, in pre-Plinian 
times, the upper parts of the mountain of Somma always collected 
the waters which at lower levels, where the inclination was still 
great, produced enormous erosion on the northern slopes, as observa- 
tions in the field clearly demonstrate. But at the same time, after 
the great crater of the Atrio, after Phase VI., had been filled up, 
all the water falling on it would either sink in or escape over the 
lower or southern edge, and not being endowed with that momentum 
acquired in falling from steep slopes above, could denude little, and 
would rather soon deposit its rocky burden. Thus there is every 
evidence that even in pre-Plinian times nature was trying to obliterate 
that want of symmetry produced by the grand explosive eruption of 
Phase VI. and their predecessors. 
The author then again states his opinion that the old cone of 
Somma was not a regular one, since he considers that denudation 
during many centuries of eccentric eruptions (here the author no 
doubt means those small outbursts with outflow of lava and formation 
of parasitic cones similar to the 1861 eruption) would have destroyed 
that regularity. I have already stated my facts for believing that the 
Somma cone was a regular one, and that so far as denudation goes, 
that agent of destruction may be entirely neglected, for in an active 
volcano of the kind under consideration, we should have a surface 
similar to that of the Vesuvian cone at present, that is, not susceptible 
of any important change from meteoric agencies. Falling rain 
immediately sinks into the porous lapilli, and scorie, its energy at 
the moment of impact being chiefly converted into heat. It is only 
years or centuries after a cone of this kind has become extinct, that 
vegetation commences to grow on the upper parts of the mountain, 
and so by rendering the surface less permeable and therefore fit to 
serve as a water collector. Then, the rain retained on the surface, 
and rushing down the slopes with considerable momentum, may 
commence to erode valleys. Nevertheless, the symmetry of a cone 
is not destroyed, for the valleys that score its sides are roughly 
equal in size, etc., and symmetrically arranged around it, like the 
surface between the ribs of a half-opened umbrella. 
As to eccentric lava eruptions, the only way in which they modify 
the symmetry of the cone would be in raising parasitic cones upon 
its flanks; but surely, the author can hardly deem these worthy of 
consideration in the question now being considered. Would the 
numbers of cones and craterets on Etna in any way influence the 
formation of a crater of explosion ? 
But to still more strongly confirm what I have asserted, we will 
choose a part of Monte Somma still uncovered by lavas, and apply 
the contour-line method. he locality I shall choose is the two 
strips of the mountain which constitute the Bosco Cognoli, very 
lately and only in part covered by lava, and which bears a few 
degrees §. of 8.H. from the present axis. It is therefore evident that 
as this is at an angle of at least 40° with the line of displacement 
of the eruptive axis, the calculated results of eccentricity should be 
