opment of volcanic forces to suppose the movement which modified
the shape of the cone to have been intermittent and gradual, and not to
have consisted of a single effort, or one sudden and violent
convulsion.[540]

_Vesuvian lavas._--The lavas of Somma are characterized by containing
disseminated crystals of leucite (called, by the French, amphigene), a
mineral said to be very rare in the modern lavas of Vesuvius, which are
in general much more scoriaceous and less crystalline than those of
Somma.[541]

At the fortress near Torre del Greco a section is exposed, fifteen feet
in height, of a current which ran into the sea; and it evinces,
especially in the lower part, a decided tendency to divide into rude
columns. A still more striking example may be seen to the west of Torre
del Annunziata, near Forte Scassato, where the mass is laid open to the
depth of twenty feet. In both these cases, however, the rock may rather
be said to be divided into numerous perpendicular fissures, than to be
prismatic, although the same picturesque effect is produced. In the
lava-currents of Central France (those of the Vivarais, in particular),
the uppermost portion, often forty feet or more in thickness, is an
amorphous mass passing downwards into lava irregularly prismatic; and
under this there is a foundation of regular and vertical columns; but
these lavas are often one hundred feet or more in thickness. We can
scarcely expect to discover the same phenomenon in the shallow currents
of Vesuvius, where the lowest part has cooled more rapidly, although it
may be looked for in modern streams in Iceland, which exceed even those
of ancient France in volume.

Mr. Scrope mentions that, in the cliffs encircling the modern crater of
Vesuvius, he saw many currents offering a columnar division, and some
almost as regularly prismatic as any ranges of the older basalts; and he
adds, that in some the spheroidal concretionary structure, on a large
scale, was equally conspicuous.[542] Brieslak[543] also informs us that,
in the siliceous lava of 1737, which contains augite, leucite, and
crystals of felspar, he found very regular prisms in a quarry near Torre
del Greco; an observation confirmed by modern authorities.[544]

_Effects of decomposition on lavas._--The decomposition of some of the
felspathic lavas, either by simple weathering, or by gaseous emanations,
converts them from a hard to a soft clayey state, so that they no longer
retain the smal