Page:A Treatise on Geology, volume 2.djvu/251

 CHAP. IX. were seen by Neapolitan fishermen; and on the 18th of July, a British man of war passing near the spot, white columns were seen in the sea, rising from a dark hillock, which threw up stones and ashes. It was then judged by captain Swinburne to be 70 or 80 yards in diameter, and about 20 feet high. In August it had grown to a circumference of 3240 feet, its height being 107 feet; and in the middle was a crater 780 feet in circumference; the columns of ashes rose to a height of 3000 or 4000 feet. The evolution of gases was inconsiderable. When examined, the mass of the island was found to be a dark vesicular lava, with a few fragments of limestone, and other non-volcanic rocks. On the 28th of September, according to Prevost, the circumference of the island was 2300 feet, and the height from 100 to 230 feet. In the winter of 1831-2, its loose and perishable fabric had yielded to the action of the waves, and disappeared from the surface. It is now a dangerous shoal, shelving gradually to the deep sea bed (100 fathoms), out of which it originally sprung; on the neighbouring parts of the sea bed, probably, a considerable deposit of volcanic sediment is spread. Such is the history of the vanished island of Sciacca.

In the Azores, in 1628, an island rose from 160 fathoms water, in 15 days, to a height of 360 feet above the sea; Mr. De la Beche has found in the MS. of the Royal Society, a notice of another island, which had been thrown up in 1690, but soon afterwards, like Sciacca, was dissolved and sunk again in the sea.

In 1811, off St. Michael's, in the same group of islands, a volcano was observed to be active in the sea, on the 13th of June. On the 17th it shot up black columns of cinders to the height of 700 or 800 feet above the sea, and at other times clouds of vapour; the eruptions being accompanied by great noises and vivid lightnings. On the 4th of July, the island which was formed was 1 mile in circumference, almost circular, and about 300 feet high; the crater discharged hot