om the Pacific.

_Periodical alternation of Earthquakes in Syria and Southern Italy._--It
has been remarked by Von Hoff, that from the commencement of the
thirteenth to the latter half of the seventeenth century, there was an
almost entire cessation of earthquakes in Syria and Judea; and, during
this interval of quiescence, the Archipelago, together with part of the
adjacent coast of Lesser Asia, as also Southern Italy and Sicily,
suffered greatly from earthquakes; while volcanic eruptions were
unusually frequent in the same regions. A more extended comparison,
also, of the history of the subterranean convulsions of these tracts
seems to confirm the opinion, that a violent crisis of commotion never
visits both at the same time. It is impossible for us to declare, as
yet, whether this phenomenon is constant in this and other regions,
because we can rarely trace back a connected series of events farther
than a few centuries; but it is well known that, where numerous vents
are clustered together within a small area, as in many archipelagoes for
instance, two of them are never in violent eruption at once. If the
action of one becomes very great for a century or more, the others
assume the appearance of spent volcanoes. It is, therefore, not
improbable that separate provinces of the same great range of volcanic
fires may hold a relation to one deep-seated focus, analogous to that
which the apertures of a small group bear to some more superficial rent
or cavity. Thus, for example, we may conjecture that, at a comparatively
small distance from the surface, Ischia and Vesuvius mutually
communicate with certain fissures, and that each affords relief
alternately to elastic fluids and lava there generated. So we may
suppose Southern Italy and Syria to be connected, at a much greater
depth, with a lower part of the very same system of fissures; in which
case any obstruction occurring in one duct may have the effect of
causing almost all the vapor and melted matter to be forced up the
other, and if they cannot get vent, they may be the cause of violent
earthquakes. Some objections advanced against this doctrine that
"volcanoes act as safety-valves," will be considered in the sequel.[488]

The northeastern portion of Africa, including Egypt, which lies six or
seven degrees south of the volcanic line already traced, has been almost
always exempt from earthquakes; but the northwestern portion, especially
Fez and Morocco, which fall wi