IxXXiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



considerable elevation, but a bill consisting of tuff alone, as appears 

 to be tbe case with a large part at least of Rocca Monfina, could only- 

 have attained its present height in consequence of some elevatory 

 movement subsequent to its ejection ; and if this be admitted, we 

 have before us, in the central trachytic rock of Monte della Croce, 

 an agent calculated to cause such an upheavement, and itself hardly 

 to be accounted for without such a supposition." 



To this theory of the formation of Rocca Monfina, it appears to 

 me that the following objections may fairly be urged. In the first 

 place, the advocates of the theory of elevation have never yet been 

 able to give a satisfactory answer to the general objection, so fre- 

 quently urged, that if, by the application of a force from beneath, a 

 series of horizontal sedimentary deposits were suddenly elevated, so 

 as to form a conical mountain with a central cavity or crater, the 

 brim of that crater could not be continuous ; it must necessarily be 

 rent by numerous fissures, that would be widest at the brim, become 

 gradually narrower towards the base, and finally disappear. Now 

 Dr. Daubeny and M. Pilla both describe the brim of the original cra- 

 ter, for a large part of the circumference, as unbroken — as "absolutely 

 intact^ In the second place, if the trachytic cone raised up the 

 horizontal beds of tuff, although it is very possible that it might be 

 in a pasty state and might protrude, still the beds of tuff it raised up 

 would lie upon its sides all round. Now we are told, that not only 

 is there no tuff on the Monte de Santa-Croce, but that it stands iso- 

 lated in the centre of an area, with its base more than three-quarters 

 of a mile from the inner escarpment of the crater. The height of 

 the escarpment above the interior plain from which the Monte de 

 Santa-Croce rises is not given ; but whatever it is, the removal of the 

 whole mass of materials from the space between the conical hill and 

 that escarpment, has to be accounted for by the elevation theory ; 

 and it is not easy to conceive by what agency this could have been 

 effected. 



It would not be at all contrary to what has been seen in other 

 volcanic mountains, if we suppose that the external cone was formed, 

 in great part, by materials erupted from a central vent ; that after 

 a repose of some duration, there should be a subsidence in the in- 

 terior, leaving Rocca Monfina in the same state as Vesuvius is 

 represented to have been in the time of Strabo, as shown in the 

 figure that accompanies Dr. Daubeny' s memoir ; that subsequent 

 volcanic action, that had been long dormant, should again burst 

 through the old vent, the weak part of the incumbent mass ; and that 

 lava in a pasty state should be protruded, which would settle into a 

 conical form, if the whole of Monte de Santa-Croce be so composed. 



Dr. Daubeny, in arguing in favour of the elevation theory, adverts 

 to the marine fossil shells found on the sides of Somma, as a proof 

 that that mountain was formed by the upheaval of sedimentary de- 

 posits. If such remains had been found in a continuous bed, the 

 evidence would not have been conclusive, for Somma may have been 

 a cone formed by submarine eruptions, since raised in mass ; but it 

 is admitted that the shells are found in loose blocks in the tuff, and 



