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in the crater almost to the very edge, and threatened to overflow and inundate the surrounding country, when they were withdrawn by a subterranean canal cut in the rock, and poured into the Tiber by a connecting stream.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 Æneid, 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.

The volcanic origin of the Roman Forum suggests the curious idea of the intimate connection of some of the greatest events of history with volcanic centres. Where the strife of nature has been fiercest, there by a strange coincidence the storm of human passion has been greatest. The geological history of a region is most frequently typical of its human history. We can predicate of a scene where the cosmical disturbance has been great,—where fire and flood have contended for the mastery, leaving the effects of their strife in deepening valleys and ascending hills,—that there man has had a strangely varied and eventful career. The strongholds and citadels of the earth, where the great battles of freedom and civilisation have been fought, were all untold ages previously the centres of violent plutonic disturbances. Edinburgh Castle, enthroned on its trap-rock, once the centre of a volcano, is associated with the most stirring and important events in the history of Scotland; Stirling Castle rises on its trap-rock erupted by volcanic action above a vast plain, across which a hundred battles have swept; Dumbarton Castle, crowning its trappean promontory, has represented in its civil history the protracted periods of earthquake and eruption concerned in the formation of its site; while standing in solitude amid the stormy waters of the Firth of Forth, the Bass Rock, once a scene of fiery confusion, of roaring waves and heaving earthquakes, has formed alternately the prison where religious liberty has been strangled, and the fortress where patriotism has taken its last stand against the forces of the invader. Palestine, Greece, Italy, Switzerland, and Scotland, the countries that have had the most remarkable history, and have done most to advance the human race, are distinguished above other countries for their geological convulsions and revolutions. The Roman Forum is thus but one specimen among numerous others of a law of Providence which has associated the strife of nature with the strife of man, and caused the ravages of the most terrible elements to prepare the way for the highest development of the human race.