But the most extraordinary of all the volcanic phenomena within the historical period was the sudden rising on two memorable occasions of the waters of the Alban Lake, which now lie deep down within the basin of an extinct crater.  The first swallowed up the royal palace of Alba, and was so sudden and violent that neither the king nor any of his household had time to escape.  The other occurred during the romantic siege of the Etruscan city of Veii, near Rome, by Camillus, four hundred years before Christ.  The waters on that occasion rose two hundred and forty feet in the crater almost to the very edge, and threatened to overflow and inundate the surrounding country, when they were withdrawn by a subterranean canal cut in the rock, and poured into the Tiber by a connecting stream.  This emissary, which may still be seen, was constructed owing to a hint given by an Etruscan soothsayer, that the city of Veii would not be captured till the Alban Lake was emptied into the sea.  The deep winding cavern on the face of the Aventine Hill, said to have been inhabited by the monstrous giant Cacus, the son of Vulcan, who vomited fire, and was the terror of the surrounding inhabitants, was evidently of volcanic origin; and the local tradition from which Virgil concocted his fable was undoubtedly derived from a vivid recollection of the active operations of a volcano.  When Evander, as described in the eighth AEneid, conducted his distinguished guest to the top of the Tarpeian Rock, in after ages so famous as the place of public execution, and composed of very hard lava, he assured him that an awful terror possessed the place, and that some unknown god had his abode there.  The shepherds said it was Jupiter, and that they had often seen him kindling his lightnings and hurling his thunderbolts from thence.  Evander then pointed to the ruined cities of Saturnia and Janiculum, on either side of the Tiber, whose destruction had been caused by the wrath of the god.  There can be no doubt that this fable clothed with supernatural colouring some volcanic phenomena which had taken place on this spot during the human period.  Even as late as three hundred and ninety years after the foundation of Rome, a chasm opened in the Forum, and emitted flames and pestilential vapours.  An oracle declared that this chasm would not close until what constituted the glory of Rome should be cast into it.  Marcus Curtius asked if anything in Rome was more precious than arms and valour; and arraying himself in his armour, and mounting on a horse splendidly equipped, he leapt in the presence of the Roman people into the abyss, when it instantly closed for ever.  We thus see that the geology of the Roman plain throws no inconsiderable light upon the early history and traditions of the Eternal City, and brings within the cycle of natural phenomena what were long supposed to be purely fabulous incidents, the inventions of a poetic imagination.  I have dwelt upon these geological incidents so fully, because nowhere does one realise the striking contrast between the shortness of man’s existence on earth, as in places like the Roman plain, where the traces of cosmical energy have been greatest and most enduring.