Page:Popular Science Monthly Volume 20.djvu/62

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.
52
THE POPULAR SCIENCE MONTHLY.

up by alternate ejections of fluid lava and fragmentary materials. The slopes of their sides are subject to a wide range of variation, corresponding with the varying character and degree of liquidity of these materials. The sides of the cones are liable to be rent asunder and traversed with fissures, through which liquid lava forces its way and

PSM V20 D062 Outlines of the summit of vesuvius in the eruption of 1767.jpg
Fig. 9.—Outlines of the Summit of Vesuvius during the Eruption of 1767.

gives rise to new subsidiary cones, or series of cones, along the lines of the clefts. These cones are called parasitic cones, and frequently attain considerable dimensions, some of those on the flanks of Etna being nearly eight hundred feet high. A typical example of a group of such cones is given in the Island of Ischia, where several parasitic