Another very extraordinary outburst, this time on land, took place in 1538 on the opposite side of the Bay of Naples to where Vesuvius stands. There, on the shores of the Bay of Baiæ, a mountain 440 feet high was built up in one week, where all had before been quiet in the memory of man. For two years before the outburst came, rumblings and earthquakes had alarmed the people, and at last one day the sea drew back from the shore and the ground sank about fourteen feet, and then on the night of Sunday, September 29, 1538, it was hurled up again, and steam, fiery gases, stones, and mud burst forth, driving away the frightened people from the village of Puzzuoli about two miles distant. For a whole week jets of lava, fragments of rock, and showers of ashes were poured out, till they formed the hill now called Monte Nuovo, 440 feet high and measuring a mile and a half round the base. And there it has remained till the present day, perfectly quiet after the one great outburst had calmed down, and is now covered with thickets of stone-pine trees.
These sudden outbursts show that some great change must occur in the state of the earth's crust under the spots where they take place, and we know that eruptions may cease for centuries in any particular place and then begin afresh quite unexpectedly. Vesuvius was a peaceable mountain overgrown with trees and vines in the time of the Greeks till in the year A.D. 79 occurred the terrific outburst which destroyed Herculaneum and Pompeii, shattering old Vesuvius to pieces, so that only the cliffs on the northwest remain and are called Somma (see Fig. 37), while the new Vesuvius has grown up in the lap, as it were, of its old self. Yet when we visit the cliffs of Somma, and examine the old lava streams in them, we see that the ancient peaceful mountain was itself built up by volcanic outbursts of molten rock, and showers of clinkers or scoriæ, long before man lived to record it.
Meanwhile, when once an opening is made, we can understand how after an eruption is over, and the steam and lava are exhausted, all quiets down for awhile, and the melted rock in the crater of the mountain cools and hardens, shutting in once more the seething mass below. This was the state of the crater when I saw it in 1864, though small streams still flowed out of two minute cones; but since then at least one great outburst had taken place in 1867, and now on this November night, 1868, the imprisoned gases rebelled once more and forced their way through the mountain-side.