205 
ON THE SOLFATARA, ETC., OF NAPLES. 
In. the hills behind the Solfatara, and in the caverns in the 
hills where the hot air issues, there is also abundant proof of 
chemical action. The hills, once composed of tuff, are now 
converted into a beautiful white earth, a large part of 
which is an admirable (violin, or porcelain earth, capable of 
making the very finest varieties of porcelain, such as was once 
manufactured on the hill behind Naples, and is known to 
collectors under the name Capo di monti. The kaolin is still 
dug, though not in large quantities. Besides this there are 
large incrustations of common alum (a double silicate of 
alumina and potash), and the whole Solfatara crater has been 
inclosed with a view of manufacturing alum, taking advan- 
tage of the materials at hand and the heat of the steam jet. 
The caverns are entirely lined with exquisitely beautiful 
crystals, yellow, brown, and white, of which sulphur and the 
sulphates seem to form the principal part. I did not observe, 
however, any crystals or efflorescence of pure sulphur. The 
hills on the other side, near Pisciarello, are cut into at various 
points artificially, and are laid bare by the torrents that 
occasionally come down them. The rock is white and soft, 
and might certainly be more made use of than it is. It is 
also coated with aluminous crystals. 
From the evidence afforded by the colour and composition 
of the rocks, it seems certain that the volcanic force under the 
Gulf of Naples, though it has chiefly found vent for the last 
several centuries in the immediate vicinity of Vesuvius, has 
not deserted the district between Naples and Ischia, where it 
once, perhaps, had its chief seat, and which, geologically, 
seems to be its principal axis. Naples is built in the broad 
valley between an old crater whose walls are pierced by the 
celebrated grotto of Posilipo, and the old crater of Monte 
Somma. Both are very old, using the term in reference to 
human dates and history, though, geologically, the phenomena 
are very modern. The crater to the west is, probably, the 
oldest. Just as Monte Somma has been broken by V esuvius, 
so has this unnamed crater been destroyed by that of the Lake 
of Agnano, and that again by the disturbance that produced 
the Astroni and the Solfatara, both of which are much more 
perfect. Still newer and of known date, though hardly more 
perfect, is the crater of Monte Nuovo, formed about three 
centuries ago. It is small, and exhibits at present no signs 
whatever of chemical action. But the Solfatara, though so much 
older as a crater, is, as I have pointed out, in that state of ac- 
tivity which shows the near vicinity of hot lava, and the line of 
activity certainly extends the whole distance from Naples to 
Ischia, though, probably, not approaching the surface except 
at that point. It may be useful to mention that the levels of 
