80 H. J, JOHNSTON-LAVIS ON THE GEOLOGY 
normal condition that it is undistinguishable from the Cretaceous 
Apennine rock of Castellamare, and on the other entirely converted 
into a mass of silicates. Thus we may find hand-specimens that 
show all the details of stratification, folding, and faulting of the 
lamin, and at another point its conversion into a saccharoidal snow- 
white marble with bands of tremolite (?), wollastonite, pyroxene, &c. 
Very rarely one may find a specimen of a light grey, compact, true 
trachyte, probably derived from older volcanic deposits through which 
the eruptions of Monte Somma burst their way. 
Lastly, there is an interesting series of brecciated rocks which 
have the appearance of old tufas rich in limestone fragments. These 
are so altered that the interstitial matrix may be converted into a 
pyroxenic mass, which, however, may contain a most varied series of 
minerals. The enclosed fragments of limestone are metamorphosed 
on the surface, and in some cases replaced by cavities filled with 
crystals of e wollastonite, pyroxene, olivine, and other silicates. These 
altered breccias graduate into compacted tufas of doubtful origin, 
which exhibit all stages of metamorphosis *. 
It has been my endeavour to give here nothing more than a rough 
enumeration of this interesting series of often ambiguous rocks, not 
as of use to the mineralogist, but simply for the geological lessons 
they teach us. 
The powerful abrasion to which these ejected blocks were sub- 
jected when erupted must have resulted in the detachment of many 
of their crystals, scattering them loosely through the deposif, or 
leading to their envelopment in the pumice. In this manner we 
meet with a large number of biotite and pyroxene crystals which are 
usually more or less broken. 
This deposit was first generally, but correctly, described by Breis- 
lak; but the earliest really minute examination of its components 
was made by Scacchi T, and it is described by him in these words :-— 
‘‘Lapilli of pumice containing large quantities of limestone, various 
in structure and composition, and containing all the minerals, with 
finely powdered substance, and also with fossiliferous shells ;” and, 
again, when refuting the statements of Lippi, he redescribes it, to- 
gether with similar deposits of the Plinian eruption, adding—“ The 
limestone fragments are often enveloped in the pumice, the lava 
lapilli, and loose crystals of augite, olivine, and leucite (?)” 2. 
Remarks.—The study of this deposit opens up to our comprehension 
the magnificence and majesty of the eruption to which it was due, 
and initiates us into some of the secrets of the most important phe- 
nomena of volcanic activity. 
The same thing is repeated, and the illustration is far morevivid than 
* About eighteen months since I met with a block of leucilitic lava contaming 
@ mass Of metallic iron and a sulphide, possibly troilite. This block isnow under- 
going investigation, and is likely to be of great interest in the results obiaimed. 
t ‘Lezioni di geologia’ (Napoli, 1843), pp. 171 and 172, and ‘ Notizie geolo- 
poe dei vuleani della Campania.’ 
* Osservazioni critiche sulla maniera come_fu seppelita |’ antica Pompei” 
Lettera estratta dal Bolletino archeologico Napolitano, No. vi. Marzo, 1843. 
——— SS eee 
