J. W. Judd — On Volcanoh. 349 



activity along this great volcanic band present many features in 

 common ; especially in the abundance of leucite and the group of 

 minerals allied to it ; there are also not a few points of peculiar 

 interest in connexion with these rocks which have been very admirably 

 treated by Professor voni Rath in his " Geognostiche-mineralogische 

 Fragmente aus Italien." Without, however, staying to dwell upon 

 these subjects, we shall proceed to notice the proofs which exist of 

 the occurrence of those volcanic outbursts of extraordinary violence 

 or duration to which we have referred, and which have resulted in 

 the production of some of the most marked and striking of the 

 physical features of the district. 



The frequency of the occurrence of lakes in volcanic districts is a 

 circumstance that is familiar to all geologists. Sometimes, as in the 

 case of the Lac de Chambon in the Mont Dore, the throwing up of 

 a series of volcanic cones in the midst of a valley has arrested the 

 drainage, and given rise to the formation of a lake ; in other cases, 

 precisely similar effects have resulted from the influx of a great 

 current of lava across a line of drainage. There are not wanting 

 proofs, also, that those local subterranean movements to which 

 volcanic districts are especially subject have frequently so altered the 

 levels along a line of river- valley as to lead to the damming up of the 

 stream, and to the consequent production of lakes. In all these cases 

 the lakes have been formed by the joint action of aqueous and igneous 

 forces. But there are also many examples of lakes the basins of 

 which clearly owe their origin to the action of igneous causes alone. 

 Such are the well-known Maare of the Eifel, and those numerous 

 depressions common in almost all volcanic districts, which are 

 evidently old craters that have become filled with water. 



But lying to the northward of Rome we find two lakes of such 

 vast proportions — the Lago di Bracciano being 6| miles in diameter, 

 and the Lago di Bolsena 10 miles — that we may at the first sight of 

 them be fairly led to hesitate in referring their formation to the 

 ordinary explosive action of volcanos. Dr. Daubeny, indeed, appears 

 to have been so staggered by their enormous size, that he found it 

 impossible to accept their volcanic origin. In the present chapter we 

 purpose to notice those features presented by them which appear to 

 place their mode of formation beyond question. 



In seeking to illustrate the characters and to account for the pro- 

 duction of these vast craters, it will be well to refer, in the first 

 instance, to examples of a precisely similar kind, though on a 

 somewhat smaller scale, the mode of origin of which it is not 

 possible to doubt. Vesuvius presents us with a great encircling 

 crater, that of Somma, which has a diameter of two miles and a half, 

 and which was produced during the grand paroxysmal outburst of 

 a.d. 79. There seems to be now no room for doubt that at the period 

 of this grand eruption, concerning which we possess such interesting 

 historical details, the original cone of Somma was completely gutted, 

 and that vast cavity formed in the midst of which the existing cone 

 of Vesuvius was subsequently built up. Here, then, we have an 

 illustration of the effects which may be produced by a single eruption 



