Page:Popular Science Monthly Volume 20.djvu/61

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51
VOLCANOES, THEIR ACTION AND DISTRIBUTION.

up and open, forming a gaping fissure, within which incandescent matter was visible. Masses of stone and vast quantities of pumice and mud were then thrown up to a great height for two days and nights, and, falling on the sides of the vent, formed a great mound, which was climbed up by adventurous persons on the third day, a quiet one. The ejections were resumed on the next day, when several persons who undertook to climb the hill were killed or injured, but ceased on the seventh or eighth day. The mass of the hill, which is four hundred and forty feet above the sea, was chiefly composed of the materials which were thrown out during the first two days and nights. It consists of scoriæ, lapilli, and dust, and is now covered with a thick growth of pines. The crater is marked by a steep, cup-shaped depression, the bottom of which is but little above the level of the sea. The district in which this mountain is situated contains a great number of hills, strikingly resembling it, some of which are larger, some smaller than it, but all so similar that "no stranger visiting the district, without previous information on the subject, would suspect the fact that, while all the other hills of the district have existed from time immemorial, and are constantly mentioned in the works of Greek and Roman writers, this particular hill of Monte Nuovo came into existence less than three hundred and fifty years ago."

The form of the cones is modified by the character of the materials thrown out, by the action of the weather, and by repeated eruptions. Loose material, scoriæ and lapilli, roll till they reach a position of rest, and leave a more or less regular cone. Very liquid lavas flow to great distances, resting at a very slight slope, as in the volcanoes of Hawaii, where, with a slope of only six or eight degrees, the mountains have a diameter of seventy miles at their base, and reach a height of

PSM V20 D061 Outlines of lava cones.jpg
Fig. 8.—Outlines of Lava-Cones. 1. Mauna Loa, in Hawaii, composed of fluid lava; 2. The Schlossberg of Teplitz, Bohemia, composed of very imperfectly fluid or viscid lava.

fourteen thousand feet. If, on the other hand, the lava is only imperfectly liquid, it tends to accumulate around the vent and form a more or less steep-sided bulbous mass, as in number two of the figure (Fig. 8). The shape of the cone may undergo changes during an eruption, as in the accompanying outlines of Vesuvius (Fig. 9). Most of the great volcanic mountains belong to the class of "composite cones," and are built