Dr. Thompfon, of Naples, on Siliccous Incruftations, 
@paque on the application of the flame of a 
Yamp ; which proceed from the multiplied | 
crevices or divifions, which in fuch cafe 
extend themlelves in every direftion in 
this fubftance: in the fame manner as 
tranfparent ice and glafs, when pounded, 
become white: which indeed Faujas de 
Saint Fond feemed not to have confidered 
when he propofes to us his difficulties 
relative to the caufe of the pearly bright- 
nefs which he defcribes in fimilar italac- 
tites, in page 330 of his ** Mineralogie 
des Volcans, 8vo. 1784.*. If, however, 
thefe divifions become exceflive, then the 
white is rendered perfectly opaque inftead 
of the fhining luftre of mother of pearl, 
and the ftalaétite too much cracked, crum- 
bles between the fingers. TI likewife ob- 
tained, in the winter, in exchange with 
the Ducal cabinet of Florence, a little 
piece of that more beautiful mamillonate 
ftalactite of the Montamiata, prefented 
to me, together with other f{pecimens, 
by the difcoverer himfelf, Profeffor Gior- 
gio Santi, of Pifa; and in March 1792, 
the refpeftable profeffor fhewed me and 
prefented to me, other pieces of this his 
pearled filiceous ftalactite, confiderably 
larger, and more beautiful than thofe I 
had, until then, met. with; and I then 
learnt, that thofe obferved by me the 
preceding autumn, in the Montamiata, 
were likewife not unknown to him. 
(5) Travelling in the year 1794, in the 
ifland of If{chia,I found many of thofe fina!] 
and moft brilliant filiceous ftala€tites, 
together with other white ones grouped 
in the veins and crevices of the pumice, 
fcattered among the porous kind of ftone 
which had been recently cut through, to 
form an afcent from Lacco to the baths 
of San Lorenzo; as alfo in the filiceous 
veins of another little rock of the fame 
fubftance near the fea at Lacco, being 
exactly that upon which there is a lime 
kiln. " I communicated thefe obfrvations 
the fame day to Abbate Breiflak, who 
collected numerous {pecimens of it for his 
friends. (6) The fame Abbate Breiflak, 
after my return to Naples, dug in a 
place ftill lower than the before mentioned 
afcent, with a view to meet With fulphur, 
which fome perfons afferted to have been 

* The pearls here defcribed by Faujas be- 
ing the fame with thofe of the Montamiata, 
and being of volcanic origin, as are likewile 
fo many others to be named hereafter, in- 
creafes the probability that the bafalts where 
the pearls ef Faujas are imbedded (‘* Glaf.: 
Hyalites of Miller), may be indeed of volcanic 
@rigin, which fome have hitherto doubted, 
MONTHLY Mac. No, xxx, 
335 
found there fome time before; he fuc- 
ceeded in his’ object, and carrying with 
him fome fpecimens of if to Naples, I 
had the pleafure to obferve likewife, in 
them, filiceous ftalaétites; thefe contained, 
however, filiceous veins, but larger, as 
well white and opaque as glaffy, in this 
cafe exiiting ina fubitance more compact. 
(7) Being now aware of the frequency of 
fuch phenomenon, and {eeking’ for it in 
the autumn of the fame year, I found 
likewife thefe filiceous incruttations femi- 
opague, and white in decompoted lava, 
which chietly forms the external fides of 
the ancient volcanic crater, now. called 
the Zolfatara di Pozzuoli; hence I con- 
jeCtured it would not be difficult to find 
the fame alfo within this crater, at prefent 
exhauited, except that there are yet fome 
tunnels in attivity, and that fulphur is 
daily formed there in abundance; return- 
ing then again in this prefent autumn, T 
found the faid phenomena under a great 
many varieties, fome of the ftalactite be- 
ing refplendent, and others variegated ; 
fome of the fpecimens were of a pale 
white colour and opaque, others fhining 
like glafs;  fometimes incieafing the 
fuperficies of the decompofed lava, at 
other times cementing the fragments of 
the fame, reducing them to the appear- 
ance of folie, and extremely hard ftone. 
At length ¥ perceived, now for the firft 
time, that on all occafions where occur 
extenfive furfaces of fuch ineruftations, 
they are expreisly thofe, which, ftretching 
themfelves out like paint, cover over and 
defend the yielding and pliable white 
earth there prevailing, which is nothing 
elfe than lava decompofed by vapours, 
and which, without this defence, would 
be carried totally away by the torrents of 
rain which fail upon it. 
In thefe filiceous fuperficies, which 
will often deceive not the eye only, but 
~ the hand armed with the hammer, fo as 
to induce the opinion, that the fubftance 
beneath the incruftation may likewife: be 
very hard; Edo not recolleé& other than 
the ancient crevices of the lava, which 
rendered it permeable to thofe vapours, 
which have now deftroyed it, by loading 
the whole fpace of this paffage with fili- 
ceolis earth, already held in folution. 
If my furprife was great, that ‘an ob- 
fervation to be regarded among the moft 
predominant of this place, fhould have 
eftaped the remark of preceding miner- 
alogifts, who exprefs themfelves with 
much warmth upon the inftruétive pheno- 
mena of the Zolfatara, Ttruft the accu- 
fation of envy ought “not to be attached 
» to 
