The extraordinary number of craters, a few of which are reported still to be active, gives evidence that the archipelago is the result of volcanic action.
The number of main craters may be about twenty-five, but there are very many small eruptive cones on the flanks of the old volcanoes.
All the other lakes of Central Italy, which are scattered through the volcanic districts west of the Apennines, are of an entirely difierent formation, and occupy deep cup-shaped hollows, which have undoubtedly at one time formed the craters of extinct volcanoes.
There are raised coral beds high up the mountains, and lava occurs in a variety of forms, even in solid flows; but all active volcanic agency has so long ceased that the craters have.
Of its nearest neighbour, and therefore scarcely belongs geographically to the group, all the islands are considerably elevated, with several extinct or quiescent craters rising from 2000 ft.
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