No. IV. — On the Solfatara of Pozzuoll 133 



the Solfatara, but sulphurous acid is a frequent production of 

 Vesuvius; and I find it stated as a general fact, unconnected 

 with the present subject of debate, on the authority of Sig. Mon- 

 ticelli and Covelli, as cited and compared by Dr Daubeny, that 

 the gases evolved by Vesuvius are similar to those of Etna, 

 Volcano, and Solfatara. We have not room to enter farther 

 upon this curious topic. 



The rock of which the mountain of La Solfatara is com- 

 posed is a compact lava, approaching in its characters to those 

 of trachyte. It is for the most part porphyritic, and contains 

 silex and iron in great quantity ; in some places the former pre- 

 dominates so much, as to give the lava the characters of horn- 

 stone, and in general it affects the magnetic needle. It can 

 serve no good purpose to detail particularly the varieties of this 

 rock, which may be seen at great length in the travels of Spal- 

 lanzani ; the only exception of importance to the features just 

 noticed is the lava stream already mentioned, forming the 

 Monte Olibano, which consists essentially of felspar, frequently 

 in a crystallized condition, and combining augite as an accidental 

 ingredient, and chiefly in the upper part of the current. Its 

 fracture is uneven and the colour ash gray, which is lighter 

 than that of most other lavas of the Solfatara ; it is covered by 

 fragments of scoriaceous tufa, * probably of the modern for- 

 mation to which we have already alluded ; and we may now 

 add, that in the seams of this substance are various vegetable 

 impressions, which appear to be nearly carbonized. These we're 

 carefully examined by Spallanzani, who pronounced them to 

 be undoubted species of alga marina, a very curious fact, which, 

 as far as I know is unexampled. 



The process of disintegration in all these solid rocks is car- 

 ried on chiefly by the action of the sulphurous acid vapours, 

 commencing by a removal of colour, then abrading the softer 

 portions, leaving a honey-combed appearance ; and when the 

 whole has crumbled to dust, which in a great measure consists 

 of silex, it gives the characteristic colour to the plain, and tends 

 to defend the interior rock from the farther influence of the 

 exhalations. In this condition it affects, according to Mr 



" DauWny, p. 171. 



