of the earth's surface. 149 



sed on these principles to expect, that the liquid substances 

 of basalt and of granite, in their progress through the rents 

 of our strata, have been exposed to congelation, like the 

 lava in the rents of Mount Vesuvius ; that their progress 

 has been arrested, and the protruding energy accumulated in a 

 similar manner, till it acquired sufficient power to break 

 through that obstacle, or through some other opposed to it ; 

 and it is manifest, however gradual and uniform the propelling 

 force may have been previous to its accumulation, that the ul- 

 timate laceration must have been performed, by a sudden and 

 violent motion, producing an earthquake at the surface, and 

 thus affording a more extensive and more satisfactory solution 

 of that tremendous Phenomenon that is furnished by the steam 

 of Mr Mitchell ; though I am ready to admit, that in the vol- 

 canic actions, the production and condensation of steam, on 

 many occasions, has produced very powerful effects. In the 

 Plutonic regions, as the restraining mass was beyond all com- 

 parison stronger, and thicker and heavier, than in the most 

 violent volcanic action, it must have exerted a power of resist- 

 ance greater in the same proportion ; consequently, the time of 

 that constraint must have been of incomparably longer dura- 

 tion, and the violence of the shock, when the fracture did take 

 place, though no less sudden, must have been incomparably 

 more powerful. 



Thus, as Vesuvius, in; the course of the middle ages, was 

 once at rest during several centuries, we have reason to presume 

 that the Plutonic action, after being suspended for several thou- 

 sand years *, should rush forward with a degree of violence pro- 

 portioned 



* This observation meets an objection urged against Dr Hutton by M. db 

 Breislac, a gentleman to whom I feel indebted for the handsome manner in 

 which he has mentioned the result of my experiments, and who, on most occa- 

 sions, has treated our views with peculiar fairness, but who seems in this im- 

 portant 



