upon the Phenomena, of Earthquakes. 263 
them. If this should be the case, and if the same strata * should 
be on fire in any places under such countries, as well as on the 
tops of the hills, all vapours, of whatsoever kind, raised from 
these fires, must be pent up, unless so far as they can open them- 
selves a passage between the strata; whereas the vapours raised 
from volcanos find a vent, and are discharged in blasts from the 
mouths of them. Now, if, when they find such a vent, they are 
yet capable of shaking the country to the distance of ten or 
twenty miles round, what may we not expect from them, when 
they are confined? We may form some idea of the force and 
quantity of these vapours from their effects : it is no uncommon 
thing to see them throw up, at once, such clouds of sand, ashes, 
and pumice stones, as are capable of darkening the. whole air, 
and covering the neighbouring country with a shower of dust,. 
of strata are also near akin to each other; they are generally found to ac- 
company each other ; they are both of them generally intermixed with, or 
accompanied by strata of iron ore; and they both of them, for the most 
pert, either contain, or are lodged amongst, the remains of vegetable bodies, 
and these remains of vegetable bodies in the aluminons earths, are fre- 
quently either wholly, or in part, converted into pyrites, or coal, or both. 
Numberless instances of this are to be met with in the aluminous shale of 
Whitby and other places. 
It is very probable, that to some stratum of this kind the fires of volcanos 
are owing; and this seems to be confirmed by the similarity of the ma- 
terials, which are thrown up or sublimated by the fires of volcanos, to the 
matter of the aluminous earths. _ Solfatara produces sulphur, alum, and sal 
ammoniac. The two former of these are very easily to be obtained from 
the aluminons earths, and, I suppose, the latter also ; at least it is procur- 
able from the soot of common fossil cuals, and probably, therefore, from 
the soot of that coaly matter which is intermixed with such earths. 
The aluminous earths, moreover, not only have several strata of iron ore 
lying in them, but they also contain a considerable proportion of iron in 
their composition. In currespondence to this, we find the ‘lavas of vol- 
canes, and other matters thrown out from thence, frequently containing a 
great deal of iron, the smali dust of them readily adhering to the magnet. 
As to the pytites of veins, I much doubt whether they ever coatain alum, 
or sal ammoniac ; at least they are very rarely found to contain cither the 
one or the other. ‘ 
* Tt may be asked, perhaps, why a stratum liable to take fire in some 
places, should not take fire throughout the whole extent of it. In answer 
to this, it may be said, that the same stratum may differ a little in the rich- 
ness of its combustible principles in different places; or, perhaps, the fre- 
quency of the fissures, either in the combustible stratum itself, or the stra- 
tum next to it, may let in so much water, as to prevent its taking fire, ex- 
cepting ina few places: but, if this once happens, the fire will not easily 
be put out again, but it will spread itself, notwithstanding the fissures that 
lie in its way, though they are filled with water ; for the matter on fire will 
be, in some degree at least, in a fluid state; and, for this reason, it must 
necessarily expel the water from the fissures, both on account of the ex- 
tension of its own dimensions by the heat, and of the weight of the super- 
incumbent earth, which, pressing it, will make it spread laterally. 
&e. 
