III. Experiments on Whinstone and Lava. By Sir 

 James Hall, Bart, F. R. S. & F. A. S. Edin. 



[Read, March 5. and June 18. 1798.] 



THE experiments defcribed in this paper were fuggefted to 

 me many years ago, when employed in (ludying the Geo- 

 logical Syjl em oi the late Dr Hutton, by the following plau- 

 fible objection, to which it feems liable. 



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

 ton to have flowed in a date of perfect fufion into their pre- 

 fent pofition ; but their internal ftructure, being univerfally 

 rough and flony, appears to contradict this hypothefis : for the 

 refult of the fufion of earthy fubftances, hitherto obferved in 

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

 vitreous character. 



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

 attend to the peculiar circumftances under which, according 

 to this theory, the action of heat was exerted. Thefe fubflan- 

 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 afcri- 

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



F 2 the 



