Ch. VIII.J ANTIQUITY OF THE CONE OF ETNA. 97 



tory of volcanos, we have some difficulty in deciding on the 

 relative probability of these hypotheses ; but if we embrace the 

 theory of explosions from below, the cavity would not con- 

 stitute a crater in the ordinary acceptation of that term, still 

 less would it accord with the notion of the so-called ' elevation 

 craters.' 



ANTIQUITY OF THE CONE OF ETNA. 



We have stated in a former volume, that confined notions 

 in regard to the quantity of past time, have tended, more 

 than any other prepossessions, to retard the progress of 

 sound theoretical views in Geology ; the inadequacy of our 

 conceptions of the eartlf s antiquity having cramped the free- 

 dom of our speculations in this science, very much in the same 

 way as a belief in the existence of a vaulted firmament once 

 retarded the progress of astronomy. It was not until Descartes 

 assumed the indefinite extent of the celestial spaces, and re- 

 moved the supposed boundaries of the universe, that just 

 opinions began to be entertained of the relative distances of 

 the heavenly bodies ; and until we habituate ourselves to con- 

 template the possibility of an indefinite lapse of ages having 

 been comprised within each of the more modern periods of the 

 earth's history, we shall be in danger of forming most erro- 

 neous and partial views in Geology. 



Mode of computing the aye of volcanos.' If history had be- 

 queathed to us a faithful record of the eruptions of Etna, and 

 a hundred other of the principal active volcanos of the globe, 

 during the last three thousand years, if we had an exact 

 account of the volume of lava and matter ejected during that 

 period, and the times of their production, we might, perhaps, 

 be able to form a correct estimate of the average rate of the 

 growth of a volcanic cone. For we might obtain a mean result 

 from the comparison of the eruptions of so great a number of 

 vents, however irregular might be the development of the 

 igneous action in any one of them, if contemplated singly 

 during a brief period. 



VOL. III. H 



