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Page:Popular Science Monthly Volume 19.djvu/62

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last spoken of acting together. In Fig. 2 are seen two varieties, one assuming a spire-like form. These may make their appearance anywhere on the volcano, but are usually situated in close proximity to the vent. Their size varies from twelve inches in height upward; generally from three to thirty feet. They are commenced in the major number of cases in the fissured crust recently formed over still-flowing lava. Here the vapor escapes in spasmodic puffs, and by its force a small quantity of lava is forced up and spread out around the aperture, which rapidly cools. It is followed by another puff and another oozing of lava above and around the aperture of the first. In this manner layer by layer is built up, thus giving an irregular, imbricated, roll-like appearance to the exterior. The surface is rapidly covered by brilliantly-colored sublimates, and the fumarole then presents a very pretty spectacle. The author lately was able to thoroughly watch the formation of such a fumarole some twenty feet high, its decadence and disintegration extending over a period of eight months. On passing the arm down the central tube. (i. e., the fumarole was extinct), it could be felt very regular and smooth, and having a pretty uniform bore of about nine inches.

After one slight eruption, the fumarole in question presented a very curious phenomenon. Immediately (about two or three seconds) after the explosion from the main vent, there came three terrific bangs, with a spout of vapor from its apex, the last one shooting out small fragments of still liquid lava.

This continued without variation for six hours that the author remained in the crater. The spire-like form may be varied according to

PSM V19 D062 Crater of deposition.jpg

Fig. 3.—Crater of Deposition.

surrounding circumstances. If the escape take place along a fissure, it will assume on occasions a miter-like form. There are many other varieties in form, depending on the variability of surrounding circumstances.

It is now necessary to draw attention to the great difference of opinion which has been expressed upon a point for which we have very little data to support either of two views of the question.

Vulcanologists were for a long time divided into two schools, which often waged war against each other with considerable fierceness. The so-called upheavalists were led by such eminent men as Von Buch, Elie de Beaumont, and Humboldt; whereas those who held the