The Geology of Monte Somma and Vesuvius, being a Study in Vulcanology

T he 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 Marigliano, 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 Nasone 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