304 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



has first been converted into tracbyte, and afterwards into the various 

 kinds of lava which characterize modern volcanos. Putting aside 

 then all analogy, even in Italy, this admission alone seems hostile to 

 the hypothesis, that the trachytic dome of Rocca Monfina should 

 have been heaved up all at once to its present altitude after the accu- 

 mulation of the scoriae and lava on its flanks. 



In recognizing the distinctions between trachytic and basaltic rocks, 

 their isolation and yet connection, Von Buch and Humboldt show 

 that occasionally in Europe, and often in South America, basalt is 

 superposed to trachyte. *' Tantot," says Humboldt, " ce sont de 

 larges coulees de basaltes souvent interrompues et formant des gradins 

 et des plateaux qui sillonnent et recouvrent ce terrain*." According 

 to Brongniart, trachyte is the granite of plutonic rocks, and is almost 

 as much extended over the surface f. 



In placing the trachytes immediately after the serpentines, and 

 before all subaerial volcanic rocks, Pilla is in accordance with all the 

 Italian geologists J. The same author, in his * Bichezza Minerale 

 delle Toscana,' p. 151, includes the trachyte of Monte Amiata 

 and the grey basalt of Radicofani under the term "piroidi," and 

 completely separates them from volcanic rocks. I have no doubt 

 that trachyte has been raised to the surface at different periods ; for 

 General La Marmora has shown that the Sardinian trachyte was 

 erupted anterior to the deposition of the subapennine strata. Other 

 trachytes, as in the Euganean Tuscany, the Papal States, and Ponza 

 Isles, are of posterior date, and probably contemporary with the 

 younger granitic rocks of Elba ; whilst the trachyte of Ischia alter- 

 nates with existing marine shells. In regard to Bocca Monfina, 

 Pilla is of opinion that its volcanic dejections were emitted during 

 what he calls the old alluvial (drift or diluvial) period, and were re- 

 aggregated and transported to distant parts by aqueous action §. 



Whether we regard the structure of its rocks or their close analogy 

 to similar masses in other parts of the world and in Italy, my con- 

 ^dction is, I repeat, that the trachyte of Bocca Monfina is the oldest 

 rock of the district, and was formed under different conditions from 

 the surrounding dejections, and under much more pressure. But how 

 and when was this mass of trachyte placed in its actual position ? For 

 even if the analogy of the Papal States were wanting, I should have 

 the greatest objection to admit that this rock could have been formed 

 under the same conditions as the surrounding loose dejections, much 

 less that it could have been so raised up into the atmosphere. 



From my own observation, indeed, I never could see any good 

 grounds for believing with Mr. Poulett Scrope|| (much as I admire 

 and subscribe to his views concerning the true volcanos of Auvergne), 



* Humboldt, Gisement des Roches, p. 335. 



t Al. Brongniart, Classification des Roches, p. 116. 



% See Trattato di Geologia, p. 13. 



§ See Saggio Comparativo, p. 10, Pisa, 1845 ; and " Osservazioni geognostiche 

 suUa parte settentrionale ed orientale della Campania," Annali Civili delle due 

 Sicilie, fasc. vi. 



II See Volcanos of Central France, p. 58 et passim. 



