New islands are formed either suddenly by the action of subterraneous fires, or gently by the deposit of the sediment of waters. Ancient historians and modern travellers relate facts on this subject which put it beyond all kind of doubt. Seneca assures us, that in his time the island Therasia appeared suddenly in the sea, to the astonishment of many mariners who beheld it. Pliny relates, that formerly thirteen islands in the Mediterranean sprung at the same instant out of the sea, and that Rhodes and Delos are the principal of them: it appears, from him, as well as Ammianus Marcellinus, Philo, and others, that these thirteen islands were not produced by an earthquake, nor by any subterraneous explosion, but that they were formerly hid under the water, which lowering left them uncovered. Delos had the name of Pelagia given to it, from having formerly belonged to the sea. Whether the origin of these thirteen islands is to be attributed to the action of subterraneous fires, or to some other cause which might occasion a sinking of the water in the Mediterranean, is uncertain. But Pliny relates, that the island Hiera, near Therasia, had been formed of ferruginous masses, and earth thrown from the bottom of the sea; and in chapter 89, he speaks of other islands formed in the like manner; but on this subject we have more clear and certain facts of later date. On the 23d of May 1707, at the sun's rising, there was seen, at some little distance from the island of Therasia, or Santorini, something like a floating rock in the sea; some persons, to satisfy their curiosity, went towards it, and found it a shoal which had issued from the bottom of the sea; it increased under their feet, and they brought with them the pumice-stone and oysters, which the rock still had attached to its surface. There was a slight earthquake at Santorini two days before this shoal appeared: it increased considerably till the 14th of June, it was then half a mile round, and from 20 to 30 feet high; the earth was white, and a little argilaceous; after that the sea became more and more troubled; vapours arose which infected the island Santorini; and on the 16th of July several rocks were seen to issue at one time from the bottom of the sea, and unite into one solid body. This was accompanied with a dismal noise, which continued upwards of two months. Flames issued from the new island, which kept increasing in circumference and height, and the violent explosions frequently threw large stones to more than seven miles distance. The island Santorini itself was deemed among the ancients as a modern production, and in 726, 1427, and 1573, it increased in size, and small islands were formed near it.[AD] The same volcano, which in the time of Seneca formed the island of Santorini, in that of Pliny produced Hiera or Volcanella, and in our time the shoal above-mentioned. [AD] See the Hist. of the Acad. 1708, page 23, &c.