OF MONTE SOMMA AND VESUVIUS. Al 
By both of these methods we get results very near the calcula- 
tion of Prof. Phillips, who estimated the height at 7000 feet. 
Il. Structure: Stratigraphy, Inthology, and Petrography. 
Era of permanent activity—A geologist who studies a volcano 
may depend on various methods for obtaining information. First, 
examination of the surface; secondly, examination of various sec- 
tions; thirdly, examination of component materials, as :— 
(a) Within the crater ; 
(6) Ravines scoring the slopes of the cone; 
(c) Irregular sections, cut by marine or subaerial denudation ; 
(d) Structure and dip ot neighbouring rocks ; 
(e) Ejected blocks ; 
(f) Artesian wells and other artificial sections. 
In the study of Monte Somma we shall call in all these, except 
those under the heading (c). We must include also the chemical and 
microscopical researches, and their comparison with recent materials, 
the result of observed and known phenomena. 
' The most convenient order we can follow is to attempt to become 
acquainted with the rocks upon which the volcano reposes, and to 
trace its development from its first appearance up to the present 
time. 
The position of Vesuvius has been so often described that it may 
be passsed over in the following words. 
The Campanian plain extends from the foot of the limestone 
range of the Apennines to the sea-coast, both of which run parallel. 
The main axis of the Apennines sends off a branch nearly at right 
angles, which extends across the plain and juts into the sea, forming 
the Cape of Sorrento and the Island of Capri. Opposite the angle 
of junction of these two ridges rises this volcano, with its majestic 
curves from the Campanian plain, forming the most southern member 
of the chain of vents on the western side of the Italian peninsula. 
The nearest exposures of limestone to the east are distant about 
ten kilometres, and to the south about eighteen. At this latter 
point the limestone dips under the Tertiary plain and does not 
reappear until after the Campi Phlegrei. This limestone has been 
encountered in the artesian well at the Royal Palace at Naples, at a 
depth of 480 metres. We have good reason to suppose that under 
Vesuvius it is not so far from the surface-level. From our calcula- 
tions of the great crater of the Atrio del Cavallo, its probable depth 
was 850 metres; we may therefore easily explain the large amount 
of altered limestone in the ejectamenta, as probably derived from 
the last 500 metres of the craterial hollow that was drilled through 
this sedimentary rock. 
Near the Cape of Sorrento (Campanella), the Cretaceous Apen- 
nine limestone is overlain by the Hocene Macigno, which consists 
chiefly of concretionary micaceous sandstones and marls ; it is absent 
in the immediate vicinity of Vesuvius. In the well at the Royal 
Palace of Naples these deposits were met with at 433 metres, being 
