236 Notices of Memoirs— 
at a high angle—sugeesting the probability of upheaval and giving- 
way of the strata through local voleanic agency, exerted at a very 
slight depth below the surface and frequently repeated. 
In quarrying the tufa as a building-material, invaluable on account 
of its cheapness (five pieces of a “roughly parallelopipidal shape, 
together measuring about two cubic feet, being worth only a franc) 
—the greatest difference is found to exist in the coefficient of resist- 
ance to pressure and crushing strain, according to the locality and 
series of beds whence the stone is procured. One point, however, 
is found in common; it was all bedded as a littoral deposit and in 
shallow sea-water; in confirmation of this, large oyster shells may 
be sometimes met with in it. Lead-grey volcanic tufa is largely 
developed at the southern entrance to the Bay of Naples, around 
Sorrento, but it is nowhere to be met with on the north side of 
Vesuvius, or beyond the region I am describing in the direction of 
Pozzuoli or in the islands of Procida or Ischia. 
Topographically the tufa hills of Posillipo and Naples continue 
in the direction of the old Camaldoli convent, in which neighbourhood 
scoriaceous and other lava is largely developed ; in other directions 
these hills have no connection with any others, for towards Vesuvius 
and in the direction of Caserto they die off gradually, and the plain 
at their foot is but slightly above the sea-level. From Cape Posillipo 
as far as the Vomero, above the Grotta di Pozzuoli, the hill is 
saddle-shaped, with a long, narrow ridge at the summit, and sloping 
down at a considerable angle on each side to the base. At the 
distance of half-a-mile from the mainland beyond Cape Posillipo is 
the little islet of Nisita, 180m. high, which is simply a small volcanic 
cone, with a crater broken down on the side looking towards the 
entrance of the bay, the rock consisting of volcanic tufa. 
It has always been a question to me whence this prodigious mass 
of tufa overlooking the Bay of Naples was ejected. Certainly not 
from Vesuvius or Monte Somma, far less from certain other volcanoes, 
such as the Solfatara, the Astruni, or Agnana, which have not the 
least topographical connection with the bills of which Iam speaking, 
and no denudation has taken place to authorize such a supposition ; 
most improbably also from Nisita, which, besides its insignificant 
proportions, occupies too eccentric a position. For my part I have 
for many a long year been led to believe that this tufa was produced 
by a volcano situated at a short distance from the Mergellina (part 
of Naples), but now submerged and no longer recognizable. Possibly 
we may be shortly able to clear up this difficulty, a most unexpected 
discovery having been recently made in the very heart of Naples, 
and one to which J am led to attribute considerable geological 
importance. 
At the present time a large main sewage drain is being driven 
through the soft tufa rock from the south-eastern part of the city 
to the vicinity of Fuorigrotta, outside the city to the north-west. 
Under the Villa Montfort, at the lower end of the Parco Grifeo, 
close to the Corso Vittorio-Emanuel, the sewer is situated at the 
level of 13m. 70cm. above the sea; 250m. farther north, close to 
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