But there is another set of causes which tends to modify the form and character of these composite, volcanic cones. As we have already pointed out, the sides of such cones are liable to be rent asunder from time to time, and the fissures so produced are injected with masses of liquid lava from below. These fissures, rent in the sides of volcanic cones, often reach the surface and eruptive action takes place, giving rise to the formation of a cone, or series of cones, upon the line of the fissure (fig. 61). Such small cones thrown up on the flanks of a great volcanic mountain are known as 'parasitic cones'; though subordinate to the great mountain mass, they may be in themselves of considerable dimensions. Among the hundreds of parasitic cones which stud the flanks of Etna, there are some which are nearly 800 feet in height. [Illustration: Fig. 61.--Diagram illustrating the formation of Parasitic Cones along lines of fissure formed on the flanks of a great volcanic mountain.] [Illustration: Fig. 62.--Outline of Etna, as seen from Catania.] [Sidenote: FORMATION OF PARASITIC CONES.] The building up of parasitic cones upon the flanks of a volcanic mountain tends, of course, to destroy its regular conical form. This may be well seen in Etna, which, by the accumulation of materials upon its flanks, has become a remarkably 'round-shouldered' mountain. (See figs. 62 and 63.) At the same time it must be remembered that materials erupted from the central vent tend to fill up the hollows between these parasitic cones, and thus to restore to the mountain its regularly conical form. [Illustration: Fig. 63.--Outline of Etna, as seen from the Val del Bronte.] [Illustration: Fig. 64.--Plan of the Volcano forming the Island of Ischia. _a, a, a._ The semi-circular crater-ring of Epomeo. _b, c, d._ Lava-currents which have flowed from the principal crater. _e, f, g, h._ Plateaux formed by ancient lava-currents. _k._ Montagnone. } _l._ Monte Rotaro. } _m._ Monte Tabor. } Parasitic cones and craters on the slopes _n._ Castiglione. } of the mountain. _o._ Lago di Bagno. } _p._ The Cremate. } _r._ Lava-stream of the Arso, which flowed from the Cremate in 1301. _x, x, x._ Raised beaches on the shores of the island, showing that it has recently undergone elevation. ] The Island of Ischia is a good example of a great volcanic cone the flanks of which are covered with numerous small parasitic cones. While the great central volcano has evidently been long extinct, and one side of its crater-wall is completely broken down, some of the small parasitic cones around its base have been formed within the historical period--one of them as recently as the year 1302. Fig. 64 is a plan of the Island of Ischia, showing the numerous parasitic cones scattered over the slopes of the principal cone. [Illustration: Fig, 65.--A primary Parasitic Cone with a secondary one at its base--Ischia.