[ 616 ] 
having both a ftronger roof over it, and being prefTed 
by a greater weight, (befide the additional weight of 
the water) will not only be lefs at liberty to expand 
itfelf, and confequently of lefs bulk, but it will alfo 
be eafily driven away towards the parts round about, 
where the fuperincumbent matter is lefs, and there- 
fore lighter. On the other hand, any vapour raifed 
from fires, where the fuperincumbent matter is 
lighter, finding a weaker roof over it, and being not 
fo eafily driven away under ftrata, that are thicker 
and heavier, will be very apt to break through, and 
open a mouth to a volcano ; and it muft neceflarily do 
this long before the fires can have fpread themfelves 
fufficiently, to be near equal to thofe which may 
fubfift in places that lie deeper. All this feems to 
be greatly confirmed by the fituation of volcanos, 
which are almoft always found on the * tops of 
mountains, and thofe often fome of the higheft in 
the world. 
78. If, then, the largeft fires are to be fuppofed 
to fubfift under the ocean, it is no wonder that the 
* Perhaps this may fupply us with a hint (if the conjecture is 
not thought extravagant) concerning the manner in which thefe 
mountains have been raifed, and why the ftrata lie generally more 
inclining from the mountainous countries, than thofe countries 
themfelves ; an appearance not eafily to be accounted for, but 
upon the fuppofition, that the upper parts of the earth reft upon 
matter, in fome degree, though not perfectly fluid, and that this 
matter is lighter than the earth that refts upon it. T his conjecture, 
however, will probably be thought lefs ftrange, if it be confldered, 
that the new iflands, formed about Santerini and the Azores, have 
fome of them been raifed from 200 to 300 yards, and upwards ; a 
height which might well enough intitle them to the denomination 
of mountains, if they had been raifed from lands not lying under 
the, ocean. [See Fig. 3.] 
moil 
