Page:Popular Science Monthly Volume 20.djvu/367

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Bourbon, where the stream, flowing over a precipice, has let t heavy pendent masses resembling the drippings of a guttering candle. Another illustration of the forms produced by very viscid lavas is afforded by the so-called "mamelons" of the island of Bourbon. The flow of rapid

PSM V20 D367 Vesuvian lava stream of 1858.jpg
Fig. 4.—Vesuvian Lava-Stream of 1858, exhibiting the Peculiar "Ropy" Surfaces of Slowly-moving Currents. (From a Photograph.)

currents is generally accompanied with the disengagement of vast quantities of steam, and this, doubtless, has much to do with the formation of the cindery projections which characterize the cooled surfaces of such streams. Little parasitic volcanoes are often formed on the top of the