Volcanos and Earthquakes. 45 



fresh supply of lava from a distance. If the afflux of water be 

 not interrupted, the exhalations of vapour may still continue, 

 of which we have already mentioned several instances. 



We may next consider how lava may be elevated from the 

 depth of a volcanic focus. The hypothesis, which ascribes 

 volcanic phenomena to the central heat, supposes that melted 

 matters exist at a certain depth. In adopting this opinion, we 

 need not assume that lava is produced by the melting of solid 

 rocks, but on the contrary, that melted matters have existed 

 since the creation of the world. f In the annexed diagram 



very high volcanos. The smallest of all, Stromholif is nearly always in ac- 

 tivity. The eruptions of Vesuvws are less frequent, although they arc still 

 more so than those of Etna and the Peak of Teneriffe. During the quiescence 

 of the latter, from 1706 to 1798, sixteen eruptions of the former took place. 

 From the colossal summits of the Andes, Cotopaxi and Tuvgurahia, an eiiip- 

 tion is observed scarcely once in a century. We may venture to state, 

 that the frequency of the eruptions of active volcanos is inversely as their 

 height and mass. After these general remarks, we may mention the cir- 

 cumstance that large lava streams, namely, such as issue from Etna and 

 VesuvinSj never flow from the crater itself, and that the quantity of the 

 melted matters is commonly inversely as the height of the fissures from 

 which the lava issues. But a lateral eruption of these two last-mentioned 

 volcanos always terminates by an emission of the ashes from the crater, 

 that is, from the summit of the mountain itself. This phenomenon has not 

 been seen on tlie Peak of Teneriffe these last hundred years. The crater was 

 most inactive during the eruption in the year 1708. Its basis did not sink, 

 v.hilst the greater or less depth of the crater of Vesurim, according to the 

 acute remark of Yon Buch, is an almost infallible sign of an impending 

 fresh eruption. Von Humboldt, p. 268. All this shews that the conditions 

 requisite for producing the greatest eifect, viz. for producing the highest 

 degree of the increased elasticity of the M'atery vapoure, are not always 

 present. 



+ On this supposition, we assume that no basalt has been produced by 

 the repeated melting of any known rock. Leonhard's Basalt Gebildc, 

 &c. Stuttgart, 1832, t. i. p. 263. / 



