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



down from the adjacent Apennines, whicli then alone constituted the 

 " terra firma" of Italy, and from the character of some of the youngest 

 of these remains (of which I spoke in my last communication) as 

 well as from the nature of the travertine, we infer that the waters 

 passed from a saline to a freshwater condition. This latter period 

 was, I believe, coseval with part of the younger pliocene deposits of 

 the sea, and the enormous changes in physical outline which have 

 occurred since that time in the south of Italy and Sicily are well- 

 known to all, and particularly through the labours of Sir Charles 

 Lyell. 



Volcanic Rocks of Latiwn. — The only apparently valid evidences 

 in favour of the former existence of true subaeriai volcanos in the 

 Papal States, are those which occ\ir in the Latian Hills to the south 

 of Rome, Vviiere Medici Spada and Ponzi have convinced themselves 

 that such phsenomena have prevailed. On examining these hills, I 

 could not, however, embrace the entire view^ of those authors ; for 

 although, from what I shall presently state, there is ground for 

 supposing that a central portion of this tract was a true volcano, 

 it appeared to me that the depressions occupied by the lakes of 

 Albano and Nemi cam.e under the sam.e general category as some of 

 the lakes north of Rome (Bolsena, Baccauo, &c.), and were therefore 

 excluded from terrestrial conditions*. Independently of their pecu- 

 liarity of mineral structure, it is stated by Spada and Ponzi, that all 

 the dejections which slope down from, the Alban Hills overlap the 

 tufas of the Campagna. One of the most striking of these is said 

 to terminate in the well-known basaltiform mass at Capo di Bove, or 

 the tower of Cecilia Metella near Rome ; but I confess I was mi- 

 able to observe anything like tire continuance of what might be called 

 a coulee from the Alban Hills to that classic spot. The rock, it is 

 true, contains melilite and p3Toxene, minerals of the Alban or Latian 

 system, together with WoUastonite and pseudo-nepheline. Difficult 

 as it is to trace the continuance of such so-called lava-currents, owing, 

 as I think, to great denudation, erosion, and change of outline sub- 

 sequent to their emission, the traveller perceives that in proceeding 

 southwards from Rome, he leaves behind him, as Ponzi pointed out to 

 me, those broad sweeps and wide undulations which principally cha- 

 racterize the Campagna, and when about nine miles from the city 

 he mounts on the rapid slopes of the Alban or Latian Hills. In com- 

 paring the ground and subsoil on which he then stands with that to 

 which he looks back, he already sees that masses of inclined dejections 

 having a certain community of character radiate from a common 



* In their theoretical profile showing the arrangement of the rock masses in 

 the Campagna of Home, Spada and Ponzi attempt many subdivisions of the de- 

 posits above the subapennine strata. Thus, their lowest lavas are trachyte and 

 tephrine basalt overlaid by the tuffs, scoris, iapilli, and puzzuOlane of the Cam- 

 pagna. These are all considered submarine ; but then they are overlaid by the 

 tufaceous marine limestones and travertines of quaternary age, and are followed 

 by tephrine basalt, sperone, and peperino, all of which are considered terrestrial, 

 as well as the younger group of tuffs, scoriae, and lapilli of the Latian volcanos. 

 These last have been alone succeeded by the relics of the Mediterranean sea and 

 modern fluviatile and lacustrine deposits. 



