Page:Popular Science Monthly Volume 23.djvu/489

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spontaneous combustion supplies the torch, and the whole phenomenon is explained.

That the lignite is in some way connected with the fiery metamorphosis of the Bad Lands we might have inferred from the fact that no lignite ever appears above the burned belts, and in a hill entirely burned the lignite is entirely wanting. The burned clay also corresponds in position to beds overlying the coal in hills immediately adjacent. But, as we have said, the fires in isolated spots are still burning; in some places wholly subterranean, smoldering and smoking, at other points readily seen both in nature and effects. We may discover the coal on one side of a broad-topped hill, and on the other we may look from the hill's summit down through gaping rifts to the same horizon and see everything molten at white heat, while hot air, smoke, and coal-gas, as from a furnace, make the region almost inaccessible. The accompanying diagrams (Figs. 2, 3) illustrate a burning

PSM V23 D489 Transverse section of burning lignite.jpg
Fig. 2.—Transverse Section of Burning Lignite—Thickness of Lignite-Bed exaggerated.

coal-bed, which has been photographed and dubbed "The Crater." Here the fire is burning out the lignite beneath a valley lying between two rounded ridges. As fast as the coal is removed by combustion

PSM V23 D489 Longitudonal section of burning lignite.jpg
Fig. 3.—Longitudinal Section of Burning Lignite—Thickness of Lignite-Bed exaggerated.

the whole section of the valley sinks, creating with respect to the stratification what geologists term a, fault. At the lowest part of the