158 EFFECTS of HEAT 
eruptions of Mount Vefuvius. Some of thefe, as marble and gyp- 
fum, are incapable in freedom of refifting the action of fire. We 
have likewife granite, fchiftus, gneifs, and ftones of every known 
clafs, befides many which have never, on any other occafion, 
been found at the furface of our globe. The circumftance of 
thefe fubftances having been thrown out, unaffected by the 
fire, proves, that it has proceeded from a fource, not only as 
deep, but deeper, than their native beds; and as they exhibit 
{pecimens of every clafs of minerals, the formation of which 
we pretend to explain, we need inquire no further into the 
depth of the Vefuvian fire, which has thus been proved to 
reach below the range of our {peculations. 
Vorcanice fire is fubject to perpetual and irregular varia- 
tions of intenfity, and to fudden and violent renewal, after 
long periods of abfolute ceffation. Thefe variations and inter- 
miffions, are likewife eflential attributes of fire as employed by 
Dr Hutton ; for fome geological {cenes prove, that the indura- 
ting caufe has acted repeatedly on the fame fubftance, and that, 
during the intervals of that action, it had ceafed entirely. 
This circumftance affords a complete anfwer to an argument 
lately urged againft the Huttonian Theory, founded on the 
wafte of heat which muft have taken place, as it is alleged, 
through the furface. For if, after abfolute ceflation, a power 
of renewal exifts in nature, the idea of wafte by continuance 
is quite inapplicable. 
Tue external phenomena of volcanoes are fufficiently well 
known; but our fubject leads us to inquire into their internal 
actions. This we are enabled to do by means of the foregoing 
experiments, in fo far as the carbonate of lime is concerned. 
SomE experiments which I formerly * laid before this So- 
ciety and the public, combined with thofe mentioned in this 
paper, 
* Edinburgh TranfaGions, Vol. V. Part I. p.60—66. 
