ISTOME XI. GRANITE AND GRANITIC PORPHYRY. 



colouring substance, to behold with astonish* 

 ment that this base judged to be uniform, is itself 

 a stone composed of two distinct substances, 

 which do not even always require the power of 

 the lens to be observable. Taking, for example, 

 a small piece of the base of antique red por- 

 phyry, and with a blow-pipe directing the flame 

 of a taper on it, it becomes brown by the first 

 blast of the fire ; and then are easily perceived 

 the small black and white grains, intermingled 

 ;iike those of granite ; and continuing the heat 

 to the fusion of the mass, the white semi-trans- 

 parent frothy vitrification of the white grains in- 

 dicates the felspar : the opake black glass pro- 

 duced by the others, announces the schorl ; this, 

 j(nore fusible, melts the first, and often encloses 

 small grains of felspar, before the fire has af- 

 fected them, and then their glasses mingle. As 

 ^o the proportion of the two substances, it dif- 

 fers ; but although I have observed them alter- 

 nately to take the predominance, the one over 

 jthe .other, in. the different masses that I have 

 jessayed, I have nevertheless found that it was 

 the felspar which most often predominated in the 

 base of antique red porphyry." 



He proceeds to observe, that what he calls 

 he ancient green serpentine, from the Italian 

 phraseology, and which is our green porphyry,, 



