still more by the consolidation of the lava in the fissure, the weak place is converted into one of exceptional strength. [Sidenote: INTRUSIVE MASSES BENEATH VOLCANOES.] As the sides of the cone are thus continually repaired and strengthened they are rendered more capable of withstanding the heaving forces acting from below, and these forces can then only find vent for themselves by again raising the liquefied lava to the central orifice of the mountain. Many volcanoes, like Etna, exhibit this alternation of eruptive action from the crater at the summit of the mountain, and from fissures opened upon its flanks, the former tending to raise the height of the volcanic pile, the latter to increase its bulk. But at last a stage will be reached when the volcanic forces are no longer able either to raise the lava up the long column of the central vent on the one hand, or to rend asunder the strongly-built and well-compacted flanks of the mountain on the other. It is probably under these conditions, for the most part, that the lavas find their way between the masses of surrounding strata and force them asunder in the way that we have already described. In the case of the more fluid basaltic lavas, as was pointed out so long ago by Macculloch, the liquefied materials may find their way between the strata to enormous distances from the volcanic centre. Such extended flat sheets of igneous rock retain their parallelism with the strata among which they are intruded over large areas, and did not probably produce any marked phenomena at the surface.