CHAPTER VIII. 



Speculations on the origin of the Val del Bove on EtnaSubsidencesAntiquity 

 of the cone of Etna Mode of computing the age of volcanos Their growth 

 analogous to that of exogenous trees Period required for the production of 

 the lateral cones of Etna Whether signs of Diluvial Waves are observable on 

 Etna. 



ORIGIN OF THE VAL DEL BOVE. 



BEFORE concluding our observations on the cone of Etna, the 

 structure of which was considered in the last chapter, we desire 

 to call the reader's attention to several questions : first, in 

 regard to the probable origin of the great valley already de- 

 scribed ; secondly, whether any estimate can be made of the 

 length of the period required for the accumulation of the great 

 cone ; and, thirdly, whether there are any signs on the surface 

 of the older parts of the mountain, of those devastating waves 

 which, according to the theories of some geologists, have swept 

 again and again over our continents. 



Origin of the Val del Bove. We explained our reasons in 

 the last chapter for not assenting to the opinion,, that the great 

 cavity on the eastern side of Etna was the hollow of an immense 

 crater, from which the volcanic masses of the surrounding walls 

 were produced. On the other hand, we think it impossible to 

 ascribe the valley to the action of running water alone ; for if 

 it had been excavated exclusively by that power, its depth 

 would have increased in the descent ; whereas, on the contrary, 

 the precipices are most lofty at the upper extremity, and dimi- 

 nish gradually on approaching the lower region of the volcano. 



The structure of the surrounding walls is such as we should 

 expect to see exhibited on any other side of Etna, if a cavity of 

 equal depth should be caused, whether by subsidence, or by 

 the blowing up of part of the flanks of the volcano, or by either 

 of these causes co-operating with the removing action of run- 

 ning water. 



