J. W. Judd — On Volcanos. 351 



Kocca Monfina of three miles, the similar crater-ring of Monte Al- 

 bano is not less than six miles in internal diameter ; and it is, more- 

 over, almost wholly composed of volcanic tuffs. In spite, however, 

 of the difference of size, no geological observer can for a moment 

 doubt that the exact identity of relation between Vesuvius and Monte 

 Cavo, and their respective encircling crater-rings, points to a simi- 

 larity in their mode of origin ; and of what that was in the case of 

 the former we have actually historical evidence. 



North of Rome rises another volcanic group — that of the Lago di 

 Bracciano. In this case we find a great circular hollow of almost 

 precisely the same dimensions as that of Monte Albano, and composed 

 of identical materials, namely, leucitic tuffs, with a few currents of 

 lava. The circular mountain group that incloses the Lago di Bracciano 

 only differs from that at Albano in the circumstance that no central 

 mountain rises in its midst. The great hollow occupied by the Lago 

 di Bracciano is nearly circular in form, and about 6^ miles in diameter. 

 The surface of the lake is 540 feet above the level of the sea ; while 

 the highest point of its surrounding wall, the hill known as the Rocca 

 Romana, rises to a further height of 1,486 feet. On its western 

 side the inclosing ring of hills has been cut through by the 

 River Arrone, which affords an outlet for the waters of the lake. It 

 appears clear that the excavation of this river valley has effected a 

 gradual lowering of the level of the Lago di Bracciano, in a manner 

 similar to what was suddenly effected, by artificial means, in the case 

 of the lakes of Albano and Nemi by the ancient Romans. A few 

 scattered outbursts of the volcanic forces have evidently taken place 

 in the immediate neighbourhood since the grand catastrophe by 

 which the vast crater was formed ; and numerous hot and mineral 

 springs all around bear witness to the fact that the igneous forces are 

 not even yet wholly extinct beneath it. 



The Lago di Bolsena is less perfectly circular in form than the 

 Lago di Bracciano; its length from north to south is 10J miles, and 

 its breadth from east to west nine miles. The lake lies in the midst 

 of a group of hills, wholly composed of volcanic rocks, which rise 

 gradually from the plains to heights of from 1,200 to 1,500 feet 

 above the sea. The surface of the waters of the lake is 962 feet 

 above the level of the Mediterranean, and the ring of hills around it 

 constitute heights for the most part from 300 to 500 feet above it. 

 Some few points in this crater-ring are, however, of considerably 

 greater elevation, as San Lorenzo on the north, Valentano on the 

 south, and Montefiascone on the south-east, which are respectively at 

 heights of 684, 780, and 985 feet above the level of the waters of the 

 lake. The last-mentioned point, however, owes its great elevation 

 to a later eruption, the town being built on the summit of a cinder- 

 cone which has been thrown up on the very edge of the crater-ring, 

 evidently at a period subsequent to its formation. Like that of 

 Bracciano. the crater-ring of Bolsena is cut through by a river- 

 valley, that of the Marta, which affords a means of escape for its 

 waters on its south-western side ; and it is clear that by the exca- 

 vation of this channel the surface of the lake has been gradually 

 lowered. 



