324 Conjectures concerning the Cause, and Observations 
‘some larger passages than the ordinary fissures of the earth, which 
seem much too narrow for that purpose; for, besides that the 
melted matter cannot be supposed to have any very great degree of 
fluidity, it must necessarily have a hard crust formed upon it, at all 
the fissures, by the long continued contact of the water contained 
in them: these impediments seem too great to be evercome by 
the difference of the specific gravities of the part that is to fall 
in, and the melted matter, which is the only cause that can tend 
to make it descend; the manner therefore, in which, I suppose, 
this event may be broupht about, is as follows: 
66. The matter of which any subterr aneous fire is composed, 
must be greatly extended * beyond its original dimensions hy the 
heat. As this will be brought about gradually, whilst the matter 
spreads itself, or grows hotter, the parts over the fire will be gra- 
dually raised ‘and bent; and this bending will, for some time, go 
on without any other consequence ; but, as the fire continues to 
increase, the earth will at last begin to be raised somewhat be- 
yond the limits of it. By this means, an annular space will be 
formed at the edges next to the fire, and surrounding it, a verti- 
cal section of which space, through a diameter of the fire, will 
be two long triangles, the shortest side or base of each lying next 
the fire, and the two longer sides being formed by the upper and 
lower strata, which will be separated for a considerablejextent, 
proportionably to the distance through which they are raised. 
~ from each other}. This space will be gradually filled with wa- 
ter, as it is formed, the melted matter being prevented from fill- 
ing it, by its want of fluidity, as well as on account of the other 
circumstances, under which it is to spread itself; for the lentor 
and sluggishness of this kind of matter is such, that, when some- 
what 
* As all bodies we are acquainted with are liable to be extended by heat, 
there can be no doubt of its being so in this case likewise ; but the matter of 
subterraneous fires is yet much more extended, than those bodies which are 
only capable of being melted into a solid glass, if we may judge of it from 
what we see of volcanos ; for the lavas, sciari, and pumice stones, thrown 
out from thence, even after they are cold, are commonly of much less spe- 
cific gravity, on account of their porous spongy texture, than the generality 
of earth, stones, &c. and they frequently are even lighter than water, which 
is itself lighter than any known fossil bodies, that compose strata in their 
natural state. 
+ In fig. 4. A is supposed to represent a vertical section of the matter 
on fire; BB, parts of the same stratum yet unkindled; CC, the two sec- 
tions of the annular space, (surrounding the fire) which is supposed to 
be filled with water, as far as the strata are separated; D, the several 
sets of earth, stones, &c. lying over the fire, which are raised a little, and 
bent, by the ‘expansiou of the matter at A. As it is not easy to represent 
the things aboye described in their due proportions, it may not be amiss, 
in order to prevent the figure here given from misleading the reader, to give 
some random measures of the several parts, such as may probably approach 
towards 
