lava by the action of the escaping steam. Some such miniature volcanoes, formed on Vesuvius in 1872, were so small that they were carried away on boards to be employed as illustrations in the lecture-rooms
of the University of Naples. When very viscous lavas are forced through fissures, they arrange themselves in concentric masses, like that represented in the illustration (Fig. 5), which is from New Zealand.
More liquid masses give rise to variously shaped structures, the bottle-shaped heaps of the "petrified fountains" (Fig. 6), or forms illustrated by the groups (Fig. 7) of small cones from Vesuvius. The contraction
of lava in cooling tends to produce fissures through the mass, breaking it up into prisms. Hence we have the columnar structure