THEORY of the EARTH. 281 



though in an extremely heated ftate. Thus, a lava in which 

 there is much calcareous fpar, when it comes to be expofed to 

 the atmofphere, or delivered from the comprefling force of its 

 confinement, efFervefces by the explofion of its fixed air ; the 

 calcareous earth, at the fame time, vitrifies with the other fub- 

 ftances : Hence fuch violent ebullition in volcanos, and hence 

 the emiffion of fo much pumice-ftone and afh.es, which are of 

 the fame nature. 



IN the body of our whinftone, on the contrary, there is no 

 mark of calcination or vitrification. We frequently find in it 

 much calcareous fpar, or the terra calcarea acrata, which had 

 been in a melted ftate by heat, and had been cryftallized by 

 congelation into a fparry form. Such is the lapis amygdaloides , 

 and many of our whinftone rocks, which contain pebbles 

 cryftallized and varioufly figured, both calcareous, filiceous, 

 and of a mixture in which both thefe fubftances form diftincl; 

 parts. The fpecimens of this kind, which 1 have from the 

 whinftone or porphyry rock of die Calton-hill, exhibit every 

 fpecies of mineral operation, in forming jafper, figured agate, 

 and marble; and they demonftrate, that this had been per- 

 formed by heat or fufion. 



I DO not mean to fay, that this demonftration is direcl: ; it is 

 conditional, and proceeds upon the fuppofition, that the bafal- 

 tic or porphyry rock, in which thofe fpecimens are found, is a 

 body which had been in a melted ftate. Now, this is a fuppo- 

 fition for which I have abundance of evidence, were it re- 

 quired ; but naturalifts are now fufficiently difpofed to admit 

 that propofition ; they even draw conclufions from this fact, 

 which, I think, they are not fufficiently warranted in doing ; 

 that is, from this appearance, they infer the former exiftence 

 of volcanos in thofe places. For my part, though I have made 

 the moft ftric"l examination, I never faw any veftige of fuch an 

 event. That there are, in other countries, evident marks of 

 volcanos which have been long extinguiihed, is unqueftionably 



N n true; 



