226 Dr Charles Daubeny on the 



height which the tuff has attained, and of the high angle at 

 which its beds are inclined. 



Alternating strata of tuff and lava may, indeed, be ima- 

 gined to build up, in the course of time, a mountain of consi- 

 derable elevation, but a hill consisting of tuff alone, as ap- 

 pears to be the case with a large part, at least, of Rocca Mon- 

 fina, could only have attained its present height, which is at 

 least 2000 feet above the sea, in consequence of some eleva- 

 tory movement subsequent to its ejection ; and if this be ad- 

 mitted, we have before us, in the central trachytic rock of 

 Monte della Croce. an agent calculated to cause such an up- 

 heavement, and itself hardly to be accounted for without 

 such a supposition.* 



I have not time at present to enter into the general dis- 

 cussion of the subject, and wall, therefore, only allude to two 

 facts relating to the volcanoes of Etna and Vesuvius, which 

 seem to shew that even there, where analogy would most 

 lead us to suppose that the whole of either mountain has been 

 formed by a succession of the same ejections which are con- 

 tinuing to increase it, the stubborn evidence of facts compels 

 us to adopt the elevatory theory. 



The first of these facts is the circumstance so lucidly point- 

 ed out by Elie de Beaumont, and substantiated by him through 

 a most laborious induction of particulars ; namely, that a 

 stream of lava, having an inclination of more than six de- 

 grees, cannot form a continuous mass, and, therefore, that 

 the upper portion of Mount Etna, which rises with an incli- 



*^ Although Rocca Monfina presents the best instance with which I am ac- 

 quainted, of a trachytic rock that has protruded through the centre of a crater, 

 without forming a current of lava extending down the external slope of the 

 mountain, yet we have something of the same kind in the crater of Astroni ; 

 and on the road from Rome to Florence we may observe, near Rosciglione, in 

 the Lake of Vico, the Lacus Cimini of antiquity, and an undoubted crater, a 

 conical hillock rising from the centre, the real structure of which is concealed 

 from the passing traveller by its being covered over with trees, but which may 

 very probably be of similar formation. 



Amongst extinct volcanoes, however, a still more striking parallel is presented 

 in Auvergne by the Puy de Dome, a conical mountain consisting of that friable 

 description of trachyte called domite, which rises from the midst of an amphi- 

 theatre of volcanic rocks of quite a different character from itself, and of a much 

 darker colour from the intermixture of augite. See my work on Volcanoes. 



