ON THE VOLCANIC PHENOMENA OF VESUVIUS. 405 
The difficulties to be encountered may be classified as follows :— 
1. Lithological characters of the rocks to be traversed. 
2. Temperature of the rocks. 
3. Exhalation of irritant and deleterious vapours and gases. 
4, Thermo-mineral waters. 
5. Depression of the Jand-level in relation to that of the sea. 
Fic. 3.—Section across M. Olibano and the Solfatara near Pozzuoli, 
Scale of 1 : 17,000. 
(North) } (South) 
& = 
= S$ i) 
q a fe 8 
g 8 2 os 
S io) s =<! 8 fi 1S 
EB s & = A a 
x =) +2) <3 a 
S : a ui = A 
S “% x 
ight 3 
a, Canals of trachyte; a’, trachyte streams; 6, scorie and other fragmentary 
ejecta filling the old craters; c, strata of incoherent submarine pozzolana ; 
d, fumaroles ; e, argillaceous tuffs with fragments of lignite, which in part 
fill the crater of the Solfatara. 
N.B.—The Bocca Grande is not really in the line of section, but slightly to the east. 
1. Geological structure of the region.—Just beyond the locality known 
as La Pietra, towards Pozzuoli, and near the mouth of the Cumana Rail- 
way tunnel, we meet with a compact yellow tuff much resembling that 
of Naples, Posilippo, and Pozzuoli. This tuff, rising very abruptly from 
the sea-level, constitutes the promontory now traversed by a short rail- 
way tunnel. Of this tuff is constituted the base of that cliff as well as 
some other masses. The same tuff reappears at the entry to Pozzuoli, 
and of which also is composed the almost peninsular hill on which 
the old town stands. These fragmentary materials erupted, and since 
indurated by decomposition, and some of their constituents altered, as 
usually occurs, have subsequently been subjected to so much marine 
erosion as to reduce them to submarine reefs and islands at a time when 
the sea was at a higher level, reaching to and eroding the foot of Monte 
Barbaro, reducing it to its present state of ruin. The waves, in fact, 
undermined the foot of Monte Barbaro both in front and at its two sides, 
actually breaching the Campiglione crater on the east. At that epoch 
the shore-line reached the Montagua Spaccata, the foot of Camaldoli, and 
covered the plain from Fuorigrotta to Bagnoli. It was this sea that de- 
posited those tuffs and pozzolanas which, abounding in shells and other 
skeletons of marine animals, now constitute in section the Starza of 
Pozzuoli, a terrace which extends from Monte Nuovo to the Cava Regia, 
interrupted by the already-mentioned tuff promontory of Pozzuoli. Other 
traces of this same sea-bottom, arranged as a terrace, rise behind the 
Lucrine Lake, near the Stufe di Nerone and behind the Bathing Esta- 
blishment of Patamia. The terrace at Stabia, mentioned in another part 
of this report, is referable to the same age. It is the shore-line of this 
sea around the ancient tuff islands and reefs that may be studied to per- 
fection at La Pietra. 
