138 Mr. G. P. Scrope on the Formation of Craters, 
and are thrown up again, become, as has already been said, 
greatly comminuted; and the source of the explosive vapours 
having sooner or later exhausted its energies, the accumulation 
of these ashes in the vent at length appears to stifle their further 
development, and quiescence for a time ensues. [I am speak- 
ing here, of course, of the case of such a paroxysmal eruption as 
I had the advantage of witnessing in 1822.] 
I have said that every crater is more or less circular in figure ; 
but, since the orifice of discharge will almost necessarily be 
opened on the least resisting point of some fissure broken through 
the solid pre-existing rocks, we might expect its section to be 
often lengthened in the direction of this fissure, and consequently 
to be rather oval than strictly circular. And this expectation is 
justified by observation. Sometimes two orifices have been 
opened upon the same fissure so near together that their craters 
or cones intersect each other. In the range of Puys of Auvergne 
and the Velay such examples are frequent. And in the eruption 
of 1850 of Vesuvius two craters were formed on the summit of 
the cone divided only by a narrow ridge; their common hori- 
zontal axis coinciding with the line of the great fissure, which 
in the preceding year had been visibly broken through the side 
of the cone towards the north-east. Sometimes aériform explo- 
sions take place from openings upon lateral fissures, and produce 
those minor, or (as they are often called) parasitic cones, of 
which several examples occur on the flanks both of Vesuvius and 
Etna. At other times the explosions are confined to the central 
vent of the volcano, the lava alone welling out, perhaps, at some 
lateral orifice. This, indeed, is the normal character of these 
phenomena. And it is this habitual predilection (as it may be 
called) of volcanic eruptions for the same identical vent, that 
occasions in so many instances the heaping-up of some vast moun- 
tain mass above and around it, subject to the occasional blowing- 
up of the central portion, to be re-formed again and again by sub- 
sequent eruptions. The result of the irregular alternation of 
these paroxysmal explosions and subsequent gradual expulsions 
of new matter is the appearance, so common in volcanic moun- 
tains, of a minor and central cone with its crater, rising within 
the circumference of some larger crater of earlier date, or in its 
immediate vicinity. The walls of the latter crater are of course 
often broken down on one or more sides (generally on the line 
of the original fissure) ;—perhaps reduced to a mere segment of 
its original circuit, by the combined operation of volcanic con- 
vulsions and aqueous erosions. Whoever will take the trouble 
to examine carefully an accurate map, on a sufficiently large 
scale, of almost any volcanic district (such, for example, as Vesu- 
vius and the Phlegrzan Fields, Etna and the Lipari Isles, the 
