‘ 
262 Conjectures concerning the Cause, and Observations 
the shocks of earthquakes, which are repeated at small intervals 
for some time. 
Section I].—53. The great frequency of earthquakes in the 
neighbourhood of burning mountains, is a strong argument of 
their proceeding from a cause of the same kind: and the ana- 
logy of several volcanos lying together in the same tract of 
country, as well as new ones breaking out in the neighbourhood 
of old ones, tends greatly to confirm this opinion; but what 
makes it still the more probable, is that peculiarity in the struc- 
ture of the earth, already mentioned. I observed before, that 
the same strata are generally very extensive, and that they com- 
monly lie more inclining from the mountainous countries, than 
the countries themselves: these circumstances make it very pro- 
bable, that those strata of combustible materials *, which break 
out in volcanos on the tops of the hills, are to be found at acon- 
siderable depth under ground in the level and low countries near 
them. 
* It has been imagined by some authors, that volcanos are produced by 
the pyrites of veins, and that they do not owe their origin to the matter of 
strata. In order to prove this, it is alleged, that volcanos are generally 
found on the tops of mountains, and that those are the places in which veins 
of pyrites are generally lodged. This argument being taken from observa- 
tions that have their foundation in nature, ought not to go unanswered. 
In the first place, then, the pyrites of veins, or fissures, are not fouud in 
suffitient quantities, or extending to a sufficient breadth, to be supposed 
capable of producing the fires of volcanos: it very rarely happens, that we 
meet with a vein or fissure five or six yards wide; and when we meet with 
such an one, yet, perhaps, not a twentieth part of it at most shall be filled 
with pyrites ; but the fires of volcanos, instead of being long and narrow, as 
if the matter that supplied them was deposited in veins, are generally round, 
and of far greater breadth than veins can be supposed to be. Mens. Bou- 
guer says, that the mouth of the volcano Cotopaxi is, at this time, five or 
six hundred fathoms wide; [see Hist. and Phil. of Earthquakes, p. 195.] 
and ‘the burning islaud that was raised out of the sea near ‘Tercera, as be- 
fore mentioned, was almost three leagues in diameter, and nearly round. 
fSee art. 29. ] 
_ Besides this, it is very difficult to conceive how any matters lodged in 
veins can ever take fire; for, excepting where the veins are extremely nar- 
row, they are almost always drowned ina very great quantity of water,which 
has free access to every part of them (x): neither are the pyrites of veins, by 
any means, so apt to take fire of themselves, as those of strata; and if, in- 
deed, there are any of them that will do so, yet they are but few in com- 
parison of those which will not: all those, which, beside iron and sulphur, 
contain copper, or arsenic, even in a very small proportion, are not at all 
subject to inflame of themselves. On the other hand, most of the pyrites 
of strata, if not all of them, have this property more or less. There are 
also twe sorts of strata, in which pyrites are lodged in the greatest abun- 
dance, that have the same property, and that frequently in as great a de- 
gree as themselves: these are coals and aluminous earths, or shale. There 
are some kinds of both these, that upon being exposed to the external air 
for a few months, will take fire of themselves, and burn, These two sorts 
of 
(k) See Note f in p. 257.—J. F. 
