20 G 
POPULAR SCIENCE REVIEW. 
the bottoms of all these craters, except the Solfatara, were 
originally very little above the sea, though they are now filled 
up by debris. The bottom of the Solfatara is 235 feet above 
the sea. 
The decomposition of the trachytic tufa by the vapours and 
gases that rise up through fumaroles, and its reduction to 
kaolin, is a fact of great interest to the mineralogist, as bearing 
on the cause of the production of this mineral from the felspars 
of granite. This result is generally attributed to the ordinary 
disintegration of particular varieties of felspar ; but the absence 
of kaolin in the trachytic tufa, except when acted on in the way 
above described, would seem to render it likely that disintegra- 
tion alone is not enough. Perhaps in Cornwall and other 
places where the china clays are obtained, it may be found that 
similar chemical action has taken place. 
There are no fumaroles in the district east of Naples, except 
in the great crater of Vesuvius, and close to the lavas recently 
erupted. I noticed a few in the lava of 1857, at whose vents 
were deposits of common salt ; but in the craters from which 
the late eruption took place, although there is much evi- 
dence of recent chemical action, there is very little appear- 
ance of hot air and present alteration. This is the more curious 
in connection with the condition of the Solfatara and the country 
adjacent. The small craters in the slopes of Vesuvius, as they 
contain few new fumaroles, also show but little alteration in 
the material of which they are composed. All the Vesuvian 
lavas are known to be rich in the alkaline bases, especially 
soda, and they differ in this respect from the Etna lavas. 
This does not prevent the metamorphosis being almost pre- 
cisely the same in the craters where acid vapours at a high 
temperature are evolved; but it may have something to do 
with the ultimate transformations. 
In concluding these remarks, I would remind the reader, 
that although much still remains to be worked out in the 
chemical investigation of volcanic phenomena, it is evident 
that the results already obtained are sufficiently promising to 
justify and encourage further and closer inquiries. It is only 
in this way that we can hope to understand the course of 
Nature in these her grandest operations, and, perhaps, although 
the phenomena of an actual eruption are infinitely more striking 
to the imagination, they are not on the whole more instructive 
than the continued chemical changes produced by these sub- 
sequent and much smaller movements. 
