n 



rr 



■~ s& 







Olel 



to tie 



I nil,,] 1 



s'i less 



the 

 idiff, 



gi?at 



erent 

 of tlese 



floiV. 



t'\ is tlie 

 uie pred- 

 whicli tte 



Ilu 



C tUii 



fix>m tie 

 iTC. Tte 



■ iTued bj 



^ of tlie 

 oftk 



or 



illtliiD 



(TS 



D 



for tte 



he L-^^'; 



a of 



:lies 



ame 



.<^ae^ 



■<.a O'^^. 



n 







f 



tity 







in 



uity 



Ch. XXVI.] 



PROBABLE AGE OF THE MOUNTAIN. 



43 



ence of a vaulted firmament once retarded tlie progress of 

 astronomy. It was not until Descartes assumed the indefinite 

 extent of tlie celestial spaces^ and removed tlie 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 contemplate the possi- 

 bility of an indefinite lapse of ages having been comprised 

 within each of the modern periods of the earth's history^ we 



of forming most erroneous and partial 



shall be in danp*er 



views in geology. 



If history had bequeathed to us a faithful record of the 

 eruptions of Etna, and 100 other of the principal active 

 volcanos of the globe, during the last 3,000 years, — if we had 

 an exact account of the volume of lava and matter ejected 

 during that period, and the times of their production, 

 might, perhaps, be able to form a correct estimate of the 

 average rate of the growth of a volcanic cone. 



■we 



For we might 



number 



comparison 



development 



sions. 



imes 



contemplated singly during a brief period. 



It would be necessary to balance protracted periods of in- 

 action against the occasional outburst of paroxysmal explo- 



we should have evidence of a repose of 

 seventeen centuries, like that which was interposed in Ischia, 

 between the end of the fourth century b. c. and the beginning 

 of the fourteenth century of our era."^ Occasionally a tre- 

 mendous eruption like that of JoruUo or that of Papandaj^ang 

 and others alluded to at page 11, would be recorded, giving 

 nse at once to a new mountain, or to the truncation of an 

 ancient cone, or to some vast lateral cavity like the Val del 



Bove. But the 



com 



rarity of such catastrojahes 



exalts our conception of the great duration of the intervals 

 01 rest which occur between eras of joaroxysmal violence. 



If we desire to approximate to the age of Etna, we ought 

 nrst to obtain some data in regard to the thickness of matter 

 ^nich has been added during the historical era, and then 



estimate 



^ See Vol. I. p. 006. 



