[ 59i ] 
break out in volcanos on the tops of the hills, are to be: 
found at a confiderable depth under ground in the level 
and low countries near them. If this lhould be the cafe, 
at all fubjedf to inflame of themfelves. On the other hand, moft of 
the pyrites of ftrata, if not all of. them, have this property more or 
lefs. There are alfo two forts of ftrata, in which pyrites are 
lodged in the greateft abundance, that have the fame property, 
and that frequently in as great a degree as themfelves : thefe are 
coals and aluminous earths, or (hale. There are fome kinds of. 
both thefe, that, upon being expofed to the external air for a few 
months, will take fire of themfelves, and bum. Thefe two forts 
of ftrata are alfo near akin to each other; they are generally found 
to accompany each other; they are both of them generally inter- 
mixed with, or accompanied by ftrata of iron ore; and they both 
of them, for the moft part, either contain, or are lodged amongft, 
the remains of vegetable bodies ; and thefe remains of vegetable 
bodies, in the aluminous earths, are frequently either wholly, or. 
in part, converted into pyrites, or coal, or both. Numberlefs, in- 
ftances of this are to be met with in the aluminous (hale of Whitby 
and other places. 
It is very probable, that to fome ftratum of this kind the fires of 
volcanos are owing; and this feems to be confirmed by the fimi- 
larity of the materials, which are thrown up or fublimated by the 
fires’ of volcanos, to the matter of the aluminous earths. Solfatara- 
produces fulphur, alum, and fal ammoniac. The two former of thefe 
are very eafily to be obtained from the aluminous earths, and, I. 
fuppofe,. the latter alfo ; at leaft it is procurable from the foot of. 
common foffil coals, and probably, therefore, from the foot of that 
coaly matter which is intermixed with fuch earths. 
The aluminous earths, moreover, not only have feveral ftrata of. 
iron ore lying in them, but they alfo contain a confiderable propor- 
tion of iron in their compofition. In correfpondence to this, we find 
the lavas of volcanos, and other matters thrown out from thence,, 
frequently containing a great deal of iron, the fmall duft of them, 
readily adhering to the magnet. 
As to the pyrites of veins, I much doubt whether they ever con- 
tain alum, or fal ammoniac; at leaft they are very rarely found to 
contain either the one or the other. 
and' 
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