It must be pointed out, however, that in two very important particulars the appearances presented by Stromboli differ markedly from those rhythmical gleams exhibited by the 'flashing-lights' of our coasts. In the first place, the intervals between successive flashes are very unequal, varying from less than one minute to twenty minutes, or even more; and in the second place, the duration and intensity of the red glow above the mountain are subject to like variation, being sometimes a momentary scarcely visible gleam, and at others a vivid burst of light which illuminates the sky to a considerable distance round.
Fig. 1.—Stromboli, viewed from the North-west, April 1874.
Fig. 2.—Map of tub Island of Stromboli. (Scale about two inches to a mile.)
GENERAL FEATURES OF THE MOUNTAIN.
Let us now draw near and examine this wonderful phenomenon of a mountain which seemingly ever burns with fire, and yet is not consumed. The general form of the Island of Stromboli will be gathered from an inspection of the plan, [fig. 2], which is copied from a map published by the Italian Government. When we land upon the island, we find that it is entirely built up of such materials as we know to be ejected from volcanoes; indeed, it resembles on a gigantic scale the surroundings of an iron furnace, with its heaps of cinders and masses of slag. The irregularity in the form of the island is at once seen to be due to the action of the wind, the rain, and the waves of the surrounding sea, which have removed the loose, cindery materials at some points, and left the hard, slaggy masses standing up prominently at others.
This great heap of cindery and slaggy materials rises, as we have said, to a height of more than 3,000 feet above the sea-level, but even this measurement does not give a just idea of its vast bulk. Soundings in the sea surrounding the island show that the bottom gradually shelves around the shores to the depth of nearly 600 fathoms, so that Stromboli is a great conical mass of cinders and slaggy materials, having a height of over 6,000 feet, and a base whose diameter exceeds four miles.
The general form and proportions of this mass will be better understood by an examination of the section, [fig. 3], which is also constructed from the materials furnished by the map of the island issued by the Italian Government. The same section, and the map, [fig. 2], will serve to make clear the position and relations of the point on the mountain at which the volcanic activity takes place. At a spot on the north-west slope of the mountain, about 1,000 feet below its summit, and 2,000 feet above the level of the sea, there exists a circular depression, the present active 'crater' of the volcano; and leading down from this to the sea there is a flat slope making an angle of about 35° with the horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep cliffs, as shown in the sketch [fig. 1], and the plan [fig. 2].
