C 59 ° ] 
tends greatly to confirm this opinion but what makes' 
it ftill the more probable, is that peculiarity in the 
fixudlure of the earth, already mentioned. I obierved- 
hefore, that the fame firata are generally very ex ten- 
five, and that they commonly lie more inclining from 
the mountainous countries, than the countries them- 
felves : thefe circumftances make it very probable,, 
that thofe * firata of combufiible materials,- which. 
break 
* I has been imagined by fome authors, that volcanos are pro- 
duced by the pyrites of veins, and that they do not owe their origin 
to the matter of firata. In order to prove this, it is alleged, that 
volcanos are generally found on the tops of mountains, and that 
thofe are the places in which veins of pyrites are generally lodged. 
This argument being taken from observations that have their 
foundation in nature, ought not to go unanfwered. In the firft 
place, then, the pyrites of veins, or fiiTures, are not found in fuf- 
ficient quantities, or extending to a fufficient breadth, to be fup- 
pofed capable of producing the fires of volcanos : it very rarely 
happens, that we meet with a vein or fiffure five or fix yards wide j. 
and when we meet with fuch an one, yet, perhaps, not a twentieth 
part of it at moft fhall be filled with pyrites ; but the fires of vol- 
canos, inftead of being long and narrow, as if the matter that fup- 
plied them was depofited in veins, are generally round, and of far 
greater breadth than veins can be fuppofed to be. Monf. Bouguer 
fays, that the mouth of the volcano Cotopaxi is, at this time, five 
or fix hundred fathoms wide; [fee Hilt, and Phil, of Earthquakes, 
p. 195 ] and the burning ifland that was raifed out of the fea near 
Tercera, as before-mentioned, was almofi: three leagues in diameter, 
and nearly round. [See art; 29.] 
Befides this, it is very difficult to conceive how any matters 
lodged in veins can ever take fire ; for, excepting where the veins 
are extremely narrow, they are almofi; always drowned in a very 
great quantity of water, which has free accefs to every part of 
them : neither are the pyrites of veins, by any means, fo apt to 
take fire of themfelves, as thofe of firata ; and if, indeed, there are 
any of them that will do fo, yet they arc but few in comparifon of 
thofe which will not: all thofe, which, befide iron and fulphur, 
contain copper, or arfenic, even in a very fjnall proportion, are not 
at 
