ON THE GHOLOGY OF MONTE SOMMA AND VESUVIUS. Po t3) 
3. The Grotocy of Monte Somma and Vusuvius, beng a Stupy w 
Vutcanotocy. By H. J. Jounsron-Lavis, Esq., M.D., F.G.5., 
&e. (Read June 20, 1883.) 
[Puats IT. ] 
ConrTENTS. 
I. External Form and Features of Monte Somma. 
Il. Structure, Stratigraphy, Lithology, and Petrography. 
Tif. Denndation. 
IV. Explanation and details of Sections. 
L. EHaiernal Form and Features of Monte Somma. 
Tue general form and outline of Monte Somma, the relation of the 
cone of Vesuvius piled around the recent eruptive axis, to that of 
the prehistoric one, have been so often alluded to by various authors, 
that the subject might appear exhausted. 
If Monte Somma is viewed from the north, for example from the 
bridge at Mariglianc *, it presents to the view certain characteristics 
of many volcanic cones. Its eastern and western flanks rise from the 
fertile Campanian plain at a gradually increasing angle, until at an 
average of 1050 metres of altitude, the outline becomes horizontal, 
presenting an irregular crenated ridge. In fact it exhibits that 
symmetry of form so characteristic of many truncated volcanic cones. 
If the observer has chosen the early morning, the sunlight falling 
obliquely across the face of the slopes brings out in all the detail 
of sharply cut light and shade, the ravines or valleys with which 
the mountain sides are scored. 
As the observer winds round the base in a westerly direction, 
the above-described apparent regularity disappears. From Naples, 
which lies very nearly west of Somma, the line of truncation is seen 
to slope from the Punta del Nasore 1137 metres, the highest point 
on the northern ridge, to the Pedimentina, 650 metres, the summit 
of the old crater-ridge on the southern side. This curve is concave 
downwards, and of irregular outline. It marks the edge of the 
great crater that was excavated by one, or a series of eruptions, 
which culminated in the first historic one, or that of a.p. 79. 
Rising from within and above this curved and inclined ridge, is 
the Vesuvian cone. 
It has been asserted by some writers on the subject, that Vesuvius 
is exactly concentric with Monte Somma. A little investigation, 
aided by the large new contour-map, will, I think, convince us to 
the contrary. It may, however, be noticed that the summit of 
Vesuvius (2. ¢. its eruptive axis), by measurement, lies about midway 
between the two extreme north and south limits of the great crater 
of the Atrio del Cavallo. We may therefore consider the modern 
* This town lies about 113 kilometres north of the present eruptive axis, or 
crater, of Vesuvius. 
D2 
