l
distinct ways, giving rise to varieties of structure essentially
different. Two of the more general of these varieties of form, the
crater-cone and the dome, are found in some districts, as in Auvergne,
side by side. The crater-cone consists of beds or sheets of ashes,
lapilli, and slag piled up in a conical form, with a central crater (or
cup) containing the principal pipe through which these materials have
been erupted; the dome, of a variety of trachytic lava, which has been
extruded in a molten, or viscous, condition from a central pipe, and in
such cases there is no distinct crater. There are other forms of
volcanic mountains, such as those built up of basaltic matter, of which
I shall have to speak hereafter, but the two former varieties are the
most prevalent; and we may now proceed to consider the conditions under
which the crater-cone volcanoes have been formed.

_Crateriform Volcanic Cones._--Of this class nearly all the active
volcanoes of the Mediterranean region--Etna, Vesuvius, Stromboli, and
the Lipari Islands--may be considered as representatives. They consist
essentially of masses of fragmental material, which have from time to
time been blown out of an orifice and piled up around with more or less
regularity (according to the force exerted, and direction of the
prevalent winds), alternating with sheets of lava. In this way mountains
several thousand feet in height and of vast horizontal extent are
formed. The fragmental materials thus accumulated are of all sizes, from
the finest dust up to blocks many tons in weight, the latter being
naturally piled around nearest to the orifice. The fine dust, blown high
into the air by the explosive force of the gases and vapours, is often
carried to great distances by the prevalent winds. Thus during the
eruption of Vesuvius in A.D. 472 showers of ashes, carried high into the
air by the westerly wind, fell over Constantinople at a distance of 750
miles.[3]

These loose, or partially consolidated, fragmental materials are rudely
stratified, and slope downwards and outwards from the edge of the
crater, so as to present the appearance of what is known as "the dip" of
stratified deposits which have been upraised from the horizontal
position by terrestrial forces. It was this excentrical arrangement
which gave rise to the supposition that such volcanic ash-beds had been
tilted up by a force acting in the direction of the volcanic throat, or
orifice of eruption. The