NOME VIII. VOLCANIC GLUTENITfi. 509 



and of some geods with a crust of brown hema- 

 tite, which seem to be the result of the infiltra- 

 tions of a marl, which is found in pieces in this 

 bricia, and which is strongly impregnated with 

 iron. 



" C 2. Bricia, formed of fragments of brown 

 porphyry, and of porphyry with a red base, with 

 parallelepiped crystals of white felspar, frag- 

 ments of white marble, surrounded in their points 

 of contact with black lineaments, which seem 

 to be the result of an aqueous dissolution, which 

 has intimately united all the parts which com- 

 pose this singular bricia. The grey lava which 

 forms its base, and which contains some grains 

 of black melted pyroxene, is so amalgamated, by 

 the assistance of calcareous infiltrations, with 

 the other parts of the bricia, that the whole 

 forms a substance capable of being polished. 



Division 3. Bricias, or volcanic tufos, formed 

 by ejections of substances reduced to pieces, to 

 grains, or to powder, sometimes carried to a dis- 

 tance by explosions and by the winds, afterwards 

 uniting, whether they fall into the s&a, or are de- 

 posited in places where the rain water consolidates 

 them, as at Pompeia, and elsewhere. 



" 1. Volcanic tufo, which owes its origin to From water, 

 showers of black and grey pumice, divided into 



