( 171 ) 



not touched by the fire, and placed among 

 them ; as has been fhewn above *. 



When I compared the effeds of the fur- 

 nace on volcanized porphyritic rocks with 

 thofe it produces on the natural, I men- 

 tioned fome red Egyptian porphyries, the 

 bafe of which I believed to be horn-ftone, 

 rather than petrofilex, relying on the ana- 

 lyfes of Bayen, as related by Delametherie, 

 of an Egyptian porphyry of the fame co- 

 lour, and fimilar to thofe on which I made 

 my experiments f. But not having then 

 been able to analyfe them, from want of 

 time, I determined to do it afterwards, and 

 give an account of the refult, in another 

 part of this work, when I fhould again have 

 occafion to fpeak of this fpecies of volcanic 

 reck. I fhall here give this analyfis, which 

 affords a confirmation that the bafe of thefe 

 porphyries is not petrofilex, as they contain 

 magnefia, which is not found in that flone. 



* See the Note at page 82, Chap. XI, in wliich I have 

 fpoken of the efTential characlers of porphyry, 

 t See Chap. XI. 



Z From 



