352 J. W. Judd—On Volcanos. 



The lake of Bolsena differs from that of Bracciano in having two 

 islands, known as Bisentina and Martana, rising in its midst. These 

 are composed of volcanic tuffs, and present the peculiar quaquaversal 

 dips so characteristic of cinder-cones. These are evidently the re- 

 mains of two small cones, which have heen thrown up on the floor 

 of the great crater, by eruptions subsequent to the great paroxysm 

 which produced its main features. 



The series of craters which we have now described possess so many 

 features in common that it is very instructive to notice such points of 

 difference as exist between them, since these may serve to illustrate 

 the various changes, both in the nature and products of their action, 

 which volcanic centres may undergo. 



In Somma we find a crater with a diameter of two miles and 

 a half, the actual formation of which is described by historians ; 

 while the materials ejected in the course of its production still lie 

 thickly over the ruins of buried cities. Within this crater a cone — 

 that of Vesuvius — has grown up, and has been in great part destroyed 

 and re-formed several times during the last eighteen centuries. In 

 Rocca Monfina a crater-ring of almost identical character, but of 

 somewhat larger dimensions and older date, has had extruded within 

 its area bosses of bulky crystalline rock, apparently of so viscid a 

 character at the time of their emission as not to be capable of being- 

 scattered in scoria?, or of flowing in lava-streams. To pass from 

 these craters to those of Monte Albano and the Lago di Bracciano 

 (of which the diameter is almost twice as great) may at first sight, 

 perhaps, present some difficulty : but if the exact correspondence of 

 all the features, except those of size, between Somma and Vesuvius' 

 on the one hand, and the outer ring and central cone and crater of 

 Monte Albano on the other hand, be considered, no one can possibly 

 doubt the similarity of their modes of origin. The contrast is 

 sufficiently obvious between what must have occurred in the case of 

 the latter volcanic group, where a central cone of vast dimensions 

 has been built up bv eruptions subsequent to the grand paroxysmal 

 outburst that gave origin to the outer crater-ring and in that of the 

 vent of Bracciano, which became quite extinct after its final 

 grand effort. In the Lago di Bolsena a paroxysm, of such violence 

 as to produce even a still larger crater, was followed by feebler out- 

 bursts, that only sufficed to form two small cinder-cones within its 

 vast circuit. 



It is not surprising that the vast size of these great lakes of 

 Bracciano and Bolsena should have led some to entertain doubts as 

 to the possibility of their having been formed in the same way as 

 ordinary craters — that is, by explosion. But if a sufficiently large 

 series of these objects be studied, it will, we think, be found im- 

 possible to draw any clear line of distinction between those of the 

 most moderate dimensions and those which attain such vast pro- 

 portions, or to ascribe to the latter any different mode of origin to 

 that which has so clearly produced the former. 



"Without passing beyond the district with which we are now im- 

 mediately concerned, the truth of this statement may be made clearly 



