42 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
therefore 47 metres thick. It would not seem that these exist be- 
neath the volcano, judging from the ejected blocks, although perhaps 
a few peculiar and rare varieties may be the metamorphosed deriva- 
tives of this Macigno. 
In the artesian well already alluded to, overlying the last, are 
215 metres of marine deposits consisting of bituminous and ligni- 
tiferous sands and marls, micaceous towards the bottom. The mica 
is probably derived from the underlying Macigno. These beds are 
fossiliferous, the shells being all of recent species (I am told). 
An examination of the ejected blocks and fragments of foreign 
rocks, especially from the beds Puasr III., Period 2, and PHAsE 
VI., Periods 1 and 4, show many of them to be in all respects 
similar to the rocks met with in the well-section. The fossil con- 
tents of these ejected blocks have been examined by Prof. Guis- 
cardi*, and out of between 60 and 70 species, only one is unknown 
in the neighbouring seas, showing the very recent date of these 
deposits underlying Monte Somma. 
In the Royal-Palace welly+ the upper 182 metres consisted of 
pumice, tufa, fragments of trachyte, and other volcanic ejectamenta, 
containing marine shells. This is confirmed by two other wells, one 
at the Piazza Vittoria, where 280 metres of this volcanic débris was 
pierced, and another lately bored at Chiatamone, which I watched 
through 180 metres always in similar materials. It is situated at the 
foot of the escarpment of Pizzofalcone, equal to another 70 metres 
of tufa. 
We have, therefore, underlying the Campanian plain in the 
immediate vicinity of Vesuvius and resting on Secondary limestone, 
three distinct formations,viz. :—EHocene sands and marls; Pleistocene, 
non-volcanic, calcareous sands and marls; and above a series of 
volcanic deposits, chiefly of marine origin. As the volcano we are 
studying reposes on these beds, its birth must have taken place in 
the latest geological time, long after other vents in its neigh- 
bourhood. 
We often read in works treating of Monte Somma and Vesuvius 
that “the lower parts” or “base” consist of tufa, and also 
that this tufa is identical with that forming the plain and other 
volcanic hills in the neighbourhood. In the first instance, if we 
understand these two expressions to mean the toe of the moun- 
tain, which is of comparatively recent origin, then such a state- 
ment is perfectly correct. On the other hand, if the base or early 
part, or skeleton, of the volcano is implied, as is evident by sections 
given and also from the context, then we must refuse to accept such 
statements as true, since we have no evidence whatever to depend 
upon. Besides, the earliest lavas &c. that caw be seen in the section 
of the Atrio are exactly similar to the latest products of eruption. 
* Fauna Fossile Vesuyviana. 
+ I must offer my sincere thanks to Monsieur Chartier, of Naples, for his 
kindness in placing at my disposal the working reports of these wells, and for 
permission to examine part of his large and valuable collection of subterranean 
geological specimens. 
