C 6 ] 
enabled to make a curious obfervation. Upon ex- 
amining the cuts and boUow ways made by currents 
ot water in the neighbeurhood of \'^criivius and of 
oilier volcanos, I liad remarked that there lay fre- 
qucntlv a ilratnm of rich foil, of more or lefs depth, 
between the matter produced by the explofion ofiuc- 
ceeding eruptions; and I was naturally led to think 
that fuch a flratum had grown in the fame manner 
as the one abovementioned over the pumice cf 
Pompeii. Where the ftratum of good foil w.iS thick, 
it was evident to me that many years had elapfed 
between one eruption and that which fucceeded it. 
I do not pretend to fay that a jud eftimate can 
be formed of the great age of volcanos from this ob- 
fervation, but fome fort of calculation rrfight be made; 
for indance, fliould an explofion of pumice cover 
again the foot under which Pompeii is buried, the 
flratum of rich foil abovementioned would certainly 
lie between two beds of pumice; and if a like acci- 
dent had happened a thoufand years ago, the flratum 
of ricli foil would as certainly have wanted much of 
its prefent thicknefs, as the rotting of vegetables, 
manure, 6cc. is ever increafing a cultivated foil. 
Whenever I find then a fuccefiion of different flrata 
of pumice and burnt matter like that which covers 
Pompeii, intermixed with flrata of rich foil, of 
greater or lefs depth, I hope I may be allowed rea- 
lonably to conclude, that the whole has been the 
produdion of a long leries of eruptions occafioned 
by fubterraneous fire. By the fize and weight of the 
pumice, and fragments of burnt erupted matter in 
thefe flrata, it is eafy to trace them up to their fource, 
which I have done more than once in the neighbour- 
hood 
