CHAPTER VIII. 
Speculations on the origin of the Val del Bove on Etna — Subsidences — Antiquity 
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 Vol 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. 
