af 
MODIFIED ly COMPRESSION. 157- 
fuppofed anciently to have taken place, accord with the exift- 
ing ftate of our globe. - 
Tue moft powerful and effential agent of the Huttonian 
Theory, is Fire, which I have always looked upon as the fame 
with that of volcanoes, modified by circumftances which muft, 
toa certain degree, take place in every lava previous to its 
eruption. 1 
Tue original fource of internal fire is involved in great ob- 
{curity ; and no fufficient reafon occurs to me for deciding 
whether it proceeds by emanation from fome vaft central re- 
-fervoir, or is generated by the local operation of fome chemi- 
cal procefs. Nor is there any neceflity for fuch a decifion : 
all we need to know is, that internal fire exifts, which no one 
can doubt, who believes in the eruptions of Mount Vefuvius. 
To require that a man fhould account for the generation of 
internal fire, before he is allowed to employ it in geology, is 
no lefs abfurd than it would be to prevent him from reafoning 
about the conftruction of a telefcope, till he could explain the 
nature of the fun, or account for the generation of light *. 
But: while we remain in fufpenfe as to the prime caufe of 
this tremendous agent, many circumftances of importance with 
regard to it, may fairly become the fubjects of obfervation and 
difcuffion. ‘ 
Some authors (I conceive through ignorance of the facts) 
have alleged, that the fire of Aitna and Vefuvius is merely fu- 
perficial. But the depth of its ation is fufficiently proved, by 
the great diftance to which the eruptive percuflions are felt, 
and ftill more, by the fubftances thrown out uninjured by fome 
eruptions 
_ * Tuts topic, however, has of late been much. urged againift us, and an unfair 
advantage has been taken of what Mr Prayrair has faid upon it. What he gave 
as mere conjecture on a fubject of collateral importance, has been argued upon as, 
the bafis and fundamental doétrine of the fyftem. 
