VOLCANOES. 227 



Homeric ages, and has served as a beacon-light to guide the 

 mariner in the Tyrrhenian Sea, loftier volcanoes have been 

 characterized by long intervals of quiet. Thus we see that a 

 whole century often intervenes between the eruptions of most 

 of the colossi which crown the summits of the Cordilleras of 

 the Andes. Where we meet with exceptions to this law, to 

 which I long since drew attention, they must depend upon 

 the circumstance that the connexions between the volcanic 

 foci and the crater of eruption cannot be considered as 

 equally permanent in the case of all volcanoes. The channel 

 of communication may be closed for a time in the case of the 

 lower ones, so that they less frequently come to a state of 

 eruption, although they do not on that account approach more 

 nearly to their final extinction. 



These relations between the absolute height and the fre- 

 quency of volcanic eruptions, as far as they are externally 

 perceptible, are intimately connected with the consideration 

 of the local conditions under which lava currents are erupted. 

 Eruptions from the crater are very unusual in many moun- 

 tains, generally occurring from lateral fissures, (as was ob- 

 served in the case of Etna, in the sixteenth century, by the 

 celebrated historian, Bembo, when a youth,*) wherever the 

 sides of the upheaved mountain were least able, from their 

 configuration and position, to offer any resistance. Cones of 

 eruption are sometimes uplifted on these fissures ; the larger 

 ones, which are erroneously termed new volcanoes, are ranged 

 together in a line marking the direction of a fissure, which is 

 soon reclosed, whilst the smaller ones are grouped together, 

 covering a whole district with their dome-like, or hive-shaped 

 forms. To the latter belong the hornitos de Jorullo,\ the 



* Petri Eembi Opuscula, (JZtna Dialogus,} Basil, 1556, p. 63 : 

 " Quicquid in ./Etnse matris utero coalescit, nunquam exit ex cratere 

 superiore, quod vel eo inscondere gravis materia non queat, vel, quia 

 inferius alia spiramenta sunt, non fit opus. Despumant flammis urgen- 

 tibus ignei rivi pigro fluxu tolas delambentes plagas, et in lapidem 

 indurescunt." 



t Sec my drawing of the Volcano of Jorullo, of its Jiornitos, and of the 

 uplifted raalpays, in my Vues de Cordilleres, pi. xliii. p 239. 



[Burckhardt states that during the twenty-four years that have inter- 

 vened since Baron Humboldt's visit to Jorullo, the liornitos have either 

 wholly disappeared, or completely changed their forms. See Avfentiialt 

 und Reisen in Mexico in 1825 und 1834.] Tr. 



Q2 



