OF MONTE SOMMA AND VESUVIUS. 103 
materials are very fine, they may enclose organic remains, chiefly 
vegetable structures. 
At the Fontana Olivello and the Lagno di Trocchia, where these 
tufas can be seen to great advantage, trunks of trees, leaves, and 
fruits are preserved. If we follow the galleries cut into this rock 
for the collection of water at the first-mentioned locality, we may 
observe a great number of hollow tubes branching out from the pas- 
sages in all directions. A careful examination of the surface of these 
shows markings derived from the mould taken by the mud envelop- 
ing the bark on the trunks of trees that once filled these hollows. 
The only remnants of woody structure now left consist of a little 
brownish powdery lignite, which, squeezed in the hand, leaves a dirty 
brown slimy mud. By breaking up the friable tufa we may obtain 
impressions of leaves and, rarely, land-snails. 
Between Cercola and St. Sebastiano, in the midst of the 1872 lava, 
a quarry has been opened in similar deposits, which are used as 
pozzolana for mortar-making. Lately I saw a house which had 
apparently been enveloped in this material, for the roof was still 
intact. A remarkable fact was the almost complete carbonization 
of the wood used in the construction, as if it had been burnt, and 
not lignitized. The same state of carbonization is observable at 
Herculaneum and Pompeii and elsewhere, where the wood had been 
seasoned and dried, as it would be, in the walls or ceilings of a house. 
It would therefore seem probable that dry and wet (7. e. green) wood 
would conduct themselves quite differently when exposed to almost 
similar conditions, as at the Olivello and Cercola, where both were 
enveloped in mud. 
The fragments of carbonized trees and their branches, which are 
commonly met with amongst the pumice, probably owe their state 
to partial burning when enveloped by the hot lapilli, since we some- 
times find intermediate stages. 
From the fact that in the lower subdivision of these tufas we may 
recognize part of the Plinian pumice, and from the frequent occur- 
rence of a band similar in character to the band of compact pumice 
and lapilli in the pisolitic tufa above Pompeii, it seems, therefore, 
quite reasonable to suppose that a large part of the deposit below 
the black pumiceous scoria is referable to the mud formed from and 
during the Plinian eruption, as at Herculaneum. 
III. Denudation. 
Agencies.—One of the most striking features of the outer surface 
of Monte Somma is the number of valleys scoring its slopes, pre- 
senting to the observer from a distance, and also in the immediate 
vicinity, a type of scenery quite unique. This is dependent upon 
the valleys differing, both in form and size, from their equiva- 
lents found on other volcanoes of Italy and elsewhere, which we 
shall see to be due to the variation of the rocks that enter into their 
composition. 
From the point of physical geography we may divide these deposits 
