THE BRACCIAXO VOLCANO 337 



mountain. It encloses a well-marked crater with the 

 flat plain of the Campo di Annibale at its bottom. 

 Eventually the central orifice came to be choked up 

 by the lava that had risen and solidified with it, and 

 as the volcanic forces still sought an outlet to the 

 surface, they were compelled to find egress at other 

 and weaker points of the volcano. At least two ex- 

 plosions took place on the old crater-rim and produced 

 the deep-sunk and singularly impressive lakes of 

 Albano and Nemi. Others broke out on the flanks 

 of the great cone. Of these, the largest is marked 

 by the crater of the Valle Arriccia, but at least two 

 dozen of smaller size have been discriminated by the 

 geologists of the Government Survey round the outer 

 slopes of the volcano. These lateral vents not im- 

 probably mark the sites of the last eruptions. 



While the Alban Mount was heaped up on the 

 southern margin of the Campagna, another independent 

 series of volcanoes rose on the northern border. The 

 Lago di Bracciano marks the position of the vent that 

 lay nearest to Rome. The huge cavity in which this 

 sheet of water lies is some six miles in diameter and 

 not improbably owes its origin to another and still 

 more stupendous explosion than that of the Alban 

 Hills. The level of the lake is 538 English feet 

 above the surface of the Mediterranean, and as the 

 water is as much as 900 feet deep, the bottom is 362 

 feet below sea-level. The crater wall still rises in the 

 Rocca Romana to a height of 1,437 feet above the 

 sea, or 900 feet higher than the lake which it encloses. 

 Numerous streams of lava have poured down the 

 outer slopes of the cone, especially on the southern 



