NOME I. COMPACT LAVA. 327 



black, while the felspar whitens ; it then acquires 

 the property of strongly acting on the magnet. 



" Most porphyritic lavas are susceptible of a 

 fine polish, which always increases the strength of 

 their colour ; they then acquire as much bright- 

 ness and beauty as natural porphyries, and may 

 be substituted for them ; only porphyries of a pur- 

 ple, and green bases, are not found among them, 

 because those two colours become black in a less 

 degree of heat than that of volcanoes." 



The most common porphyritic lava of Etna is 

 of a greyish black with white spots, the base re- 

 sembling basalt. But the work of Dolomieu 

 having been published before mineralogy had ac- 

 quired great precision, it is to be feared that he 

 has often confounded the lavas with the original 

 rocks. 



In one of his porphyritic lavas he observed 

 crystals of specular iron ; and as he also observed 

 this metal in the same state in the dross of Monte 

 Rosso, he concludes that it is formed by sub- 

 limation*. 



* Etna, 3?g. 



