J. W. Juclcl — On Voleanos. 353 



apparent. In the Campi Phlegrsei we have several beautiful examples 

 of crater-lakes, such as Agnano and Avernus. Both of these are less 

 than one mile in diameter, and there is no more room for doubting 

 their mode of origin than there is for questioning that of Astroni, 

 which is a crater with a very small lake in its midst, or indeed of 

 that of Monte Nuovo, the formation of which was actually witnessed 

 only three centuries and a half ago. But in the immediate proximity 

 of these are the precisely similar crater- rings of Pianura and the 

 Piano di Quarto, which, although having diameters of three and four 

 miles respectively, are nevertheless so precisely similar in character 

 that it is quite impossible to assign to them a different mode of origin. 



Again, the formation of the crater-ring of Somma is an event of 

 which we have authentic records, and it is impossible to doubt that 

 an eruption on even a still grander scale must have originated the 

 precisely similar crater surrounding Monte Albano ; while, if this be 

 admitted, the analogous crater-rings of Bracciano and Bolsena cannot 

 but be assigned to the operation of similar causes. 



Indeed of the recent formation of a crater of even as vast 

 dimensions as those which we have described as existing in Italy, 

 we have an example in the grand eruption of Papaudayang, in 

 Java, in 1772, by which a gulph no less than fifteen miles long 

 by six broad was originated ! 



Accepting then the conclusion that even the vast circular lakes of the 

 Italian peninsula have been formed by explosive outbursts, similar in 

 character to, but of greater intensity or duration than some of those 

 which have been recorded during the short periods to which history 

 or tradition goes back, we may proceed to ask, what are the causes 

 which have led to the production in different cases of very dissimilar 

 structures by the same explosive action? — namely, of cones like Monte 

 Nuovo and Etna, on the one hand, having comparatively small craters 

 at their summits, and of vast craters like the Piano di Quarto and the 

 Lago di Bolsena, in which the surrounding wall is of comparatively 

 insignificant bulk and elevation. In making this distinction, how- 

 ever, it must be borne in mind that no strong lime of demarcation 

 exists between the two classes of objects. Between almost perfect 

 volcanic cones, exhibiting at their summits quite insignificant craters 

 and pit-craters with scarcely a vestige of a crater- wall, examples 

 illustrating every conceivable stage of gradation may be cited. 



It is clear that, as a general rule, the formation of volcanic cones 

 must be assigned to the operations of comparatively moderate ex- 

 plosive force, either long continued or oft repeated ; while that of 

 pit-craters must be due to comparatively short, sudden, and violent 

 outbursts. 



That the cause which produces both classes of volcanic vents is no 

 other than the expansive force of bodies of steam, which are disen- 

 gaged from masses of incandescent lava rising through fissures 

 towards the surface, is a fact now universally recognized. And to 

 the geologist familiar with the appearances presented by such 

 fissures, as filled with the now consolidated materials to which they 

 gave passage, and exposed beneath what were once eruptive vents, 



DECADE II. — VOL. II. — NO. VIII. 23 



