In some cases the central vent of a volcanic scoria-cone may be filled up by subsequent ejections. A beautiful example of this kind was observed by Abich, in the case of a small cone formed within the crater of Vesuvius in 1835, and is represented in [fig. 39].
Fig. 40.—Volcanic Cones composed of Scoriæ, and breached on one side by the outflow of lava-currents.
BREACHED CONES.
Many cones formed in the first instance of scoriæ, tuff, and pumice may give rise to streams of lava, before the vent which they surround sinks into a state of quiescence. In these cases, the liquid lava in the vent gives off such quantities of steam that masses of froth or scoriæ are formed, which are ejected and accumulate around the orifice. When the force of the explosive action is exhausted, the lava rises bodily in the crater, which it more or less completely fills. But, eventually, the weaker side of the crater-wall yields beneath the pressure of the liquid mass, and this part of the crater and cone is swept away before the advancing lava-stream. Examples of such 'breached cones' abound in Auvergne and many other volcanic districts (see [fig. 40]). A beautiful example of a cone formed of pumice, which has been breached by the outflow of a lava-stream of obsidian, occurs in the Lipari Islands, at the Rocche Rosse. It is this locality which supplies the whole world with pumice (see [fig. 41]).
Fig. 41.—Campo Bianco, in the Island of Lipari. A Pumice-cone breached by the Outflow of an Obsidian Lava-current.
It is often surprising to find how volcanic cones composed of loose materials, such as tuffs, scoriæ, or pumice, retain their distinctive forms, and even the sharpness of their outlines, during enormous periods of time. Thus, in the scoria-cones which abound in the Auvergne, and were, in all probability, formed before the historical period, the sharp edges of the craters appear to have suffered scarcely any erosion, and the cones are as perfect in their outlines as though formed but yesterday. It is probable that the facility with which these cindery heaps are penetrated by the rain which falls upon them is the cause why they are not more frequently washed away.
Fig. 42.—Volcanic Cones in Auvergne which have suffered to some extent from atmospheric denudation.