[ 10 } 
year upon the fubjeft of that magnificent volcano 
Ulloa likewife mentions this circumftance of water 
attending the eruptions of volcanos in America. 
Whenever therefore I find a tufa compofed exadly like 
that which immediately covers Herculaneuiu, and 
undoubtedly proceeded from Vefuvius, I conclude 
fuch a tufa to have been produced by water mixing 
with the erupted matter at the time of an explofion 
occafioned by fubterraneous fire ; and this obfer- 
vation, I believe, will be of more ufe than any 
other, in pointing out thofe parts of the prefent terra 
frma, that have been formed by explofion. I am 
convinced it has often happened that fubterraneous 
fires and exhalations, after having been pent up and 
confined forfome time, and been the caufeof earth- 
quakes, have forced their pafiage, and in venting 
themfelves formed mountains of the matter that 
confined them, as you will fee was the cafe near 
Puzzole in the year 1538, and by evident figns has 
been fo before, in many parts of the neighbourhood * 
of Puzzole ; without creating a regular volcano. 
The materials of fuch mountains will have but 
little appearance of having been^ produced by fire, 
to any one unaccuflomed to make obfervations upon 
the different nature of volcanos. 
If it were allowed to make a comparifon between 
the earth and a human body, one might confider a' 
country replete with combuftibles occafioning ex- 
plofions (which is furely the cafe here) to be like a 
'body full of humours. When thefe humours con- 
centre in one part, and form a great tumour out of 
which they are difcharged freely, the body is lels, 
* Phil. Tranfaa. Vol. LX. p. i. 
agitated ; 
