horizon.  Up to the foot of the mountains the sea covered the vast plain; and the action of these waves of fire and steaming floods forms a natural epic of the grandest order.  Prodigious quantities of ashes and cinders were discharged from the craters; and these, deposited and hardened by long pressure under water, formed the reddish-brown earthy rock called tufa, of which the seven hills of Rome are composed.

When the sea retired, or rather when the land rose suddenly or gradually, and the volcanoes became extinct, the streams which descended from the mountains and watered the recovered land spread themselves out in numerous fresh-water lakes, which stood an hundred and fifty feet higher than the present bed of the Tiber.  In these lakes were formed two kinds of fresh-water strata—­the first composed of sand and marl; and the second, where mineral springs gushed forth through the volcanic rock, of travertine—­a peculiar reddish-brown or yellow calcareous rock, of which St. Peter’s and many of the buildings of modern Rome are composed.  We find lacustrine marls on the sides of the Esquiline Hill where it slopes down into the Forum, and fresh-water bivalve and univalve shells in the ground under the equestrian statue of Marcus Aurelius on the Capitol; while on the face of the Aventine Hill, overhanging the Tiber at a height of ninety feet, is a cliff of travertine, which is half a mile long.  The lakes which formed these deposits must have covered their sites for many ages.  At last, by some new change of level, the lakes retired, and the Tiber scooped out for itself its present channel to the sea.

When man came upon the scene we have no definite information; but numerous flints and stone-weapons have been found among the black pumice breccias of the Campagna mixed with remains of the primitive bison, the elephant, and the rhinoceros.  Human eyes must therefore have gazed upon the volcanoes of the Roman plain.  Human beings, occupying the outposts of the Sabine Hills, must have seen that plain broken up by the sea into a complicated archipelago, and beheld in the very act of formation that wonderful region destined long ages afterwards to be the scene of some of the greatest events in human history.  The Alban Hills, whose present quiet beauty, adorned with white gem-like towns, and softened with the purple hues of heaven, strikes every visitor with admiration, were active volcanoes pouring streams of lava down into the plain even after the foundation of the Eternal City.  Livy mentions that under the third king of Rome, a shower of stones, accompanied by a loud noise, was thrown up from the Alban Mount—­a prodigy which gave rise to a nine days’ festival annually celebrated long after by the people of Latium.  The remarkable funereal urns found buried under a bed of volcanic matter between Marino and Castel Gandolfo on the Alban Hills are an incontrovertible proof that showers of volcanic ashes must have been ejected from the neighbouring volcano when the country was inhabited by human beings; nay, when the inhabitants were far advanced in civilisation, for among the objects contained in the funereal urns were implements of writing.  At the close of the skirmish between the Romans and Etruscans, near Albano, in which Aruns, the son of Lars Porsenna, was slain, whose tomb may still be seen on the spot, a noise like that which Livy mentions was heard among the surrounding hills.