Observations on the Gcognostic Character of Italy. 91 



independently of tlie present arrangenient of the valley of the 

 river ; and the subsequent course of the Tiber in the valley of 

 Rome, has evidently been determined by the then existing in- 

 equalities in the ground. Yiet, before the fresh water deposites, 

 the products of volcanic agency appeared in the basin of the 

 ancient sea. The volcanoes of Italy, whose general relations to 

 its structure We have already touched on, succeed one another 

 from the frontiers of Tusany, in an evidently continuous line, 

 which here, as in so many cases, runs parallel to the nearest 

 range of mountains.* 



The environs of Rome lie between two of the most remark- 

 able centres of these volcanic ridges, all of which, with the ex- 

 ception of those in the Campanian fields, have been extinguished 

 long before the appearance of man in this country. In the N. 

 or rather in the N.W., the trachytic Monti Cimini, between 

 Viterho and Bolsena., and with them the extinguished craters of 

 Bracciuno and la Tolfa ; S.E. the basaltic Alban Hills, with 

 the heights of Frasatti and Marino, and the ancient craters of 

 Albano and Nemi. 



The changes which have happened to these mountains in the 

 formation of the Roman soil, must be of later date than the for- 

 mation of the tertiary deposites. It is certainly a striking fact, 

 for which we are indebted to Von Buch, that in the sandstone 

 heights near Rome, among the numerous fragments they in- 

 close, we never meet with the productions of the Alban Hills. 

 In vain do we look for pieces of lava, tufa, peperino, or similar 

 appearances, which are yet frequently scattered on the declivi- 

 ties of these hills. Every where here, as in the rest of Italy, the 

 masses of volcanic tufa, ancient lava streams, and the innumerable 

 minerals which derive their origin from subterranean fires, are 



• Breislak limits the volcanic district whicli is in most intimate connexion 

 with Rome, to the space between the heights of Radicofani and the Alban 

 Hills ; and it was for a long time believed that the volcanoes of Latium were 

 completely separated from those of Campania. On the other hand, it has 

 been only lately shown by Brocchi, that the volcanic line is not interrupted, 

 as it appears, at the point where the Appenine limestone reaches the Pon- 

 tine marshes. He traced numerous vestiges of volcanic rocks through the 

 valley of the Hcrniker, and found here the chain of the Ajijienines cut right 

 across by the rectilineal fissure, which traverses the upper part of the Gavi. 

 gliano. 



