1850.] MURCHISON— EARLIER VOLCANIC ROCKS OF ITALY. 287 



deed, tlie Marquis Pareto lias afforded undoubted proof by his descrip- 

 tion and sections explanatory of the structure of the tract south of 

 Viterbo*. Monte Soriano, for example, which the traveller to Rome 

 from Viterbo leaves upon his left hand, and which rises to about 3300 

 feet above the sea, is essentially a mass of trachyte, overlapped by 

 these tuffs and scoriaceous accumulations, which encumber the sides 

 of the road at L' Imposta between Viterbo and Ronciglione. These 

 latter deposits only, according to Pareto, could have had an uninter- 

 rupted communication with the atmosphere. The priority of the 

 trachyte, or its formation under different conditions, is, as I shall here- 

 after show, a point of considerable importance in its bearing on the 

 question of the origin of Rocca Monfina in the Neapolitan States. 



* For a more special description of the western part of the volcanic region of 

 the Papal States than is oifered in my sketch, I refer the reader to a memoir of 

 the Marchese L. Pareto, entitled " Osservazioni Geologiche dal Monte Amiata a 

 Roma." (Extract from the Giornale Arcadico, Roma, 1844.) This author, by an 

 appeal to natm'al sections, proves that of the volcanic rocks, as distinguished from 

 plutonic, trachyte is the most ancient, and that at points east and south-east of 

 Viterbo, that rock, resting at once on tertiary marine strata, is covered by pumiceous 

 agglomerate and volcanic tuff. All the trachytic masses which I have seen in Italy, 

 conveyed to me the idea that they had issued in a pasty state, so as to form domes 

 and flattened cones, which, traversing the pre-existing strata of subaqueous origin, 

 were the precursors of all the other volcanic productions of the peninsula. The 

 numerous varieties of trachyte, its crystals of riaccolite, its iithological divisions 

 into fire-stones, paving-stones, &c., and its raetamorphic influence on the con- 

 tiguous deposits, do not come within my present object. (See the works of 

 P. Savi, Pareto, Pilla, &c.) All the subsequent volcanic products of the Papal 

 States are grouped together by Pareto under the heads of tuft' with peperino, 

 and lava with lapilli. He believes that all the tuft* with peperino and soUd ag- 

 glomerates were formed and arranged under water, whilst the eruption and fall 

 of the lapilli may have occurred at points in connexion with the atmosphere. 

 He gives a full account of the form and structure of the dejections all around the 

 great cavity of the Lake of Bolsena, and is disposed to think, that its parasitic 

 and crateriform depressions, occupied by smaller lakes, maybe craters of elevation. 

 Although no one of these cavities — not even the Lago de Vico— seemed to me to 

 fall into the same category as the extinct true volcanos of Auvergne, still I agree 

 with Pareto, that some of the scoriaceous and pumiceous materials, particularly 

 on the plateau south of Viterbo, were probably ejected into the atmosphere. These 

 have, I admit, so subaerial an aspect, that they may well represent the last operation 

 in a series of volcanic eruptions, which terminated as the grounds rose, by throw- 

 ing up much matter into the atmosphere. Whilst Pareto believes that the depres- 

 sions near L' Imposta and the Lake of Vico may have been craters whence the 

 leuciticandhornblendic "tephrine" basalts or lavas flowed, he thinks, as I do, that 

 by far the greater portion of every volcanic eruption, even in these Colles Ciminii, 

 was subaqueous. In my opinion, such outbursts as that of Graham's Island would 

 explain all, even the most recent, of the volcanic phsenomena in the northern 

 Campagna and the region around Viterbo. 



The post-pliocene shelly formations on the coast of Civita Vecchia, which are 

 loaded with numerous fragments of the earlier volcanic rocks of which we are 

 treating, show that these last mentioned were consolidated anterior to the accu- 

 mulations of that ante-historic sea. This evidence tends also to prove, that 

 although, even at that time, so remote in respect to our day, the mollusca living in 

 the sea were the same as those which now inhabit the Mediterranean, the physical 

 geography of the coast of Italy must have been widely different from its present 

 outline ; and that as hills, that are now 300 feet above the sea, were then beneath it, 

 so all the lower countries of the Campagna must have been then under water, 

 whether salt, brackish, or fresh. This point will be again adverted to in treating 

 of the Latian volcanos. 



VOL. VI. PART I. Y 



