DOMAIN X. TRANS ILIENT. 



transparent felspar, are mistaken for quartz, in- 

 asmuch as there is one direction in which their 

 vitreous fracture is exactly like that of quartz; 

 but their fusibility easily distinguishes them, 

 when brought to the proof of the blow-pipe, 

 porphyries. " By the inverse of what we have said, the 

 best characterised porphyries easily pass to the 

 state of granite. It is enough that their base 

 shows a beginning of regular aggregation ; and 

 there are few large masses of red porphyry among 

 the most perfect, in which spots are not observed, 

 often more than a foot in extent, where the grains 

 of felspar multiply so as to touch each other; 

 little crystals of black schorl are then seen in the 

 midst of them, which have also profited by the 

 local facility given to the aggregation, or which 

 perhaps has caused it by seizing the iron ; the 

 presence of which, when it is free and oxyginated, 

 so far as to assume the red colour, seems to place 

 an obstacle to the crystallisation. Thus also are 

 these parts of granitic appearance discoloured t 

 one would often believe that those large grey 

 granitose spots, which disfigure the purple co- 

 lour of the rock, proceeded from foreign sub- 

 stances accidentally incorporated in the paste of 

 the porphyry ; if one did not discern on the 

 margin of those spots, that the grains become 

 gradually less distinct, and reassume the tissue 



