of the IJland of Ponza. 37 j 
to illuftrate what I have written on its fubjeCh In one part of 
the ifland there is a fort of tuffa, remarkably good for the pur- 
pofe of building. It is as hard as our Bath ftone, and nearly 
of the fame colour, without any mixture of fragments of 
lava or pumice ftone, which ufually abound in the tuffa’s in 
the neighbourhood of Naples, Baia, and Puzzole. 
The drawing (feeTab. XL), which is a view of the harbour 
of Ponza, will give you a very good idea of the appearance of 
the ifolated rocks of lava and bafaltes which have been fepa- 
rated, by the force of the fea, from the fofter parts of the 
ifland, and of which there are an infinite number, as you will 
fee in the exa£t geometrical plan of the ifland of Ponza (Tab. 
X.), which likewife accompanies this letter. 
When I was laft in England, I inquired of many of the 
manufacturers of glafs, whether it had ever happened, that 
the glafs cooling in their furnaces had taken any diftinCt forms 
like prifms or cryftallizations ; but I got no fatisfa&ory anfwer 
until I applied to the ingenious Mr. Parker, of Fleet-ftreet, 
who not only informed me, that, fome years ago, a quantity 
of his flint glafs had been rendered unferviceable by taking 
fuch a form in cooling ; but alfo gave me feveral curious fpeci- 
mens of the glafs itfelf: fome of them are in laminae, which 
may be eafily feparated ; and others refemble bafaltic columns 
in miniature, having regular faces. I was much pleafed with 
this difcovery, proving to me, beyond a doubt, the volcanic 
origin of moft bafaltes. Many of the rocks of lava of the 
ifland of Ponza are, with refpeCl to their configurations, 
ftrikingly like the fpecimens of Mr. Parker’s above-men- 
tioned glafs, none being very regularly formed bafaltes, but all 
having a tendency towards it. Mr. Parker could not account 
for the accident that occafioned his glafs to take the bafaltic 
forms ; 
