ig4 Effects of Heal modified hj Compression, 



Vesuvius. To require that a roan should account for the 

 generation of internal fire, before he is allowed to em-ploy it 

 in geology, is no less absurd than it would be to prevent him- 

 from reasoning about the construction of a telescope, till he- 

 could explain the nature of the sun, or account for the ge- 

 neration of light *. But while we remain in suspense as to 

 the prime cause of this tremendous agent, many circum- 

 stances of importance with regard to it may fairly become 

 the subjects of observation and discussion. 



Some authors (I conceive through ignorance of the facts) 

 have alleged, that the fire of iEtna and Vesuvius is merely 

 superficial. But the depth of its action is sufficiently 

 proved by the great distance to which the eruptive percus- 

 sions are felt, and still more by the substances thrown out 

 uninjured by some eruptions of Mount Vesuvius. Some of 

 these, as marble and gypsum, are incapable, in freedom, 

 of resisting the action of fire. We have likewise granite, 

 schistus, gneiss, and stones of every known class, besides 

 many which have never, on any other occasion, been found 

 at the surface of our globe. The circumstance of these sub- 

 stances having been thrown out unaffected by the fire, 

 proves that it has proceeded from a source, not only as 

 deep, but deeper than their native beds; and as they exhibit 

 specimens of every class of minerals, the formation of which 

 we pretend to explain, we need inquire no further into the 

 depth of the Vesuvian fire, which has thus been proved to 

 reach below the range of our speculations. 



Volcanic fire is subject to perpetual and irregular varia- 

 tions of intensity, and to sudden and violent renewal, after 

 long periods of absolute cessation. These variations and in- 

 termissions- are likewise essential attributes of fire, as em- 

 ployed by Dr. Hut ton ; for some geological scenes prove 

 that the indurating cause has acted repeatedly on the same 

 substance, and that, during the intervals of that action, it 

 had ceased entirely. This circumstance affords a complete 



* This topfc, however, has of Tate been much urged again-st us, and an 

 unfair advantage has been taken of what Mr. Playfair lias said upon it. What 

 he gave as mere conjecture on a subject of collateral importance, has been- 

 argued upon as the basis and fundamental doctrine of the system. 



answer 



