1850.] MURCHISON — EARLIER VOLCANIC ROCKS OF ITALY. 303 



tween tlie central and external rocks, this fact alone at once indicates 

 their marked difference. 



In advocating the application of the elevation-crater theory to 

 Rocca Monfina (in common with Abich and Pilla), and in separating 

 the central trachyte from the external accumulations, Dr. Daubeny 

 thus speaks of it : — " A conical mass of rock so considerable, and 

 yet so completely circumscribed within the area of the crater, could 

 only, as it would seem, have been brought into the position which it 

 is seen to occupy, by being heaved up all at once from the interior of 

 tl;ie globe whilst in a semi-fluid or pasty state, but not in a condition 

 of actual liquidity." In theorizing upon the effects of such elevation 

 of the trachyte, he says : — " We have before us an agent which would 

 be not only competent to uplift the surrounding strata of tuff, but 

 which must necessarily have done so, if the latter had been at the 

 time of its eruption in a horizontal position ; and to suppose these 

 gradually formed by successive showers of loose incoherent materials 

 before the trachytic rock in its centre was produced, seems to imply 

 a forgetfulness of the height which the tuff has attained, and the 

 high angle at which its beds are inclined." Now, in my opinion, 

 every circumstance, lithological and geological, is in favour of the 

 inference, that the central trachyte is of higher antiquity, and was 

 originally formed under different conditions than the surrounding 

 dejections. Abich describes the rock as ''trachytic dolerite," or an 

 intermede between trachyte and greenstone, which contains much 

 green augite and brown mica ; and when on the spot I could not 

 hesitate for a moment, even judging from mineral characters only, in 

 believing the central trachyte to be the oldest rock of the tract. In 

 parts it has almost the aspect of an old porphyry. Nay, Dr. Dau- 

 beny' s own description of its external aspect would have led me to 

 this conclusion. " Its surface," says he, " so far from presenting the 

 rugged aspect which volcanic rocks usually assume, is so uniformly 

 clothed with vegetation, and in such a state of complete culture, that 

 but for the amphitheatre of hills which encloses the table-land on its 

 summit, the circular form of which betrays the origin of the mountain 

 of which it forms the outer margin, no one could dream from its 

 physiognomy that the whole was of igneous formation." 



In drawing his conclusions as to the relative age of this trachyte of 

 Rocca Monfina, the geologist ought necessarily to be guided by the 

 analogy of the succession established in the Papal States, Tuscany, 

 and the adjacent Ponza Isles ; and enough has already been cited in 

 respect to the hills around Viterbo, to testify that the trachytic rocks 

 were there either the first- or the deepest-born of all the earlier vol- 

 canic products. Their order is shown by positive sections and super- 

 position. 



The inference of my friend Dr. Daubeny, that the trachyte is the 

 last formed of the rocks of Rocca Monfina, has the more surprised 

 me when I look to the good explanation he has given of the close re- 

 lations of trachytes to granites, and consequently their greater distinc- 

 tion, as I should say, from modern subaerial volcanic rocks. He 

 traces that beautiful series of transitions, by which primordial granite 



VOL. VI. PART I. Z 



