MODIFIED by COMPRESSION. 157 



fuppofed anciently to have taken place, accord with the exift- 

 ing ftate of our globe. 



The molt powerful and elfential 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, 

 to a certain degree, take place in every lava previous to its 

 eruption. 



The original fource of internal fire is involved in great ob- 

 fcurity; and no fufficient reafon occurs to me for deciding 

 whether it proceeds by emanation from fome vafl 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 mould 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 conflrudion 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 fubje&s of obfervation and 

 difcuffion. 



Some authors (I conceive through ignorance of the fads) 

 have alleged, that the fire of iEtna and Vefuvius is merely fu- 

 perficial. But the depth of its action is fufficiently proved, by 

 the great diftance to which the eruptive percuffions are felt, 

 and (till more, by the fubflances thrown out .uninjured by fome 



eruptions 



* This topic, however, has of late been much urged againft us, and an unfair 

 advantage has been taken of what Mr Playfair has faid upon it. What he gave 

 as mere conje&ure on a fubjeft of collateral importance, has been argued upon as 

 the bafis and fundamental doclrine of the fyftem. 



