Page:Popular Science Monthly Volume 20.djvu/369

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of basalt. A similar structure is exhibited in all kinds of lavas, and in other rock-masses which have been heated by contact with igneous rocks and gradually cooled. Columns of a dissimilar character are produced by the unequal cooling of different parts of the stream, so that, if the stream be thick, the lower parts will form stout, vertical columns of great regularity; while the upper part, cooling less regularly, will produce smaller and less regular columns (Fig. 8). Fingal's

PSM V20 D369 Exposed section of lava stream on the side of river ardeche.jpg
Fig. 8.—Section of a Lava-Stream exposed on the Side of the River Ardèche, in the Southwest of France.

Cave, in the Island of Staffa, has been formed in the midst of a lava-stream which has been cooled in this manner. The thick, vertical columns, which rise from beneath the level of the sea, are divided by joints and have been broken away by the action of the sea, and a great cavern has been produced, the sides of which are formed by vertical columns, while the roof is made up of smaller and interlacing ones; and the whole structure bears some resemblance to a Gothic cathedral. The columns formed in cooling vary in size from those of the Shiant Islands, near Skye, which are eight or ten feet in diameter and five hundred feet long, to the minute columns, an inch or two in length and hardly thicker than a needle, of the volcanic glasses. The larger columns are formed in slowly cooling masses. The quantity of matter that is ejected from volcanoes in the form of lava is truly enormous. Lava-streams have been described which have flowed for a distance of from fifty to a hundred miles from their source, and which have had a breadth varying from ten to twenty miles; some are five hundred feet thick, or even thicker. A mass estimated to be equal in bulk to Mont Blanc flowed out in a single eruption of Reykjanes in Iceland, in 1783. In many parts of the earth's surface, among which are tracts in our Rocky Mountain regions, successive lava-sheets have been piled upon one another to the height of several thousand feet, and cover areas of many hundred or even thousand square miles. The marks of the effects of the passage of the hot volcanic matters, with the powerful chemical agents with which they are charged, are left upon the adjoining rocks, where, for a considerable distance from the vent, limestones are converted into statuary marble, sandstones into