III. Experiments on Whinstone and Lava. By Sir 

 JAMES HALL, Bart. F. R. S. & F. A. S. Edin. 



T 



\Kead, March 5. and June 18. 1798.] 



HE experiments defcribed in tlils paper were fuggefted to 

 * me many years ago, when employed in fludying the Geo- 

 logical Syjlem of the late Dr Hutton, by the foUowmg plau- 

 fible objeaion, to which it feems liable. 



Granite, porphyry, and bafaltes, are fuppofed by Dr Hut- 

 ton to have flowed in a ftate of perfedl fufion into their pre- 

 fent pofition ; but their internal ftrudure, being univerfally 

 rough and ftony, appears to contradid this hypothefis^ for the 

 refult of the fufion of earthy fubftances, hitherto obferved m 

 our experiments, either is glafs, or poffefles, in fome'degree, the 



vitreous character. 



This objeaion, however, lofes much of its force, when we 

 attend to the peculiar circumftances under which, according 

 to this theory, the aaion of heat was exerted. Thefe fubftan- 

 ces, when in fufion, and long after their congelation, are fup- 

 pofed to have occupied a fubterraneous pofition far below what 

 was then the furface of the earth ; and Dr Hutton has afcn- 

 bed to the modification of heat, occafioned by the prefTure of 



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