the
abundance of sea-shells in Africa, near the Temple of Jupiter Ammon,
might also be the deposit of some former inland sea, which had at length
forced a passage and escaped.

But Strabo rejects this theory, as insufficient to account for all the
phenomena, and he proposes one of his own, the profoundness of which
modern geologists are only beginning to appreciate. "It is not," he
says, "because the lands covered by seas were originally at different
altitudes, that the waters have risen, or subsided, or receded from some
parts and inundated others. But the reason is, that the same land is
sometimes raised up and sometimes depressed, and the sea also is
simultaneously raised and depressed, so that it either overflows or
returns into its own place again. We must, therefore, ascribe the cause
to the ground, either to that ground which is under the sea, or to that
which becomes flooded by it, but rather to that which lies beneath the
sea, for this is more movable and, on account of its humidity, can be
altered with greater celerity.[27] "_It is proper_," he observes in
continuation, "_to derive our explanations from things which are
obvious, and in some measure of daily occurrence, such as deluges,
earthquakes, and volcanic eruptions,[28] and sudden swellings of the
land beneath the sea_; for the last raise up the sea also; and when the
same lands subside again, they occasion the sea to be let down. And it
is not merely the small, but the large islands also, and not merely the
islands, but the continents which can be lifted up together with the
sea; and both large and small tracts may subside, for habitations and
cities, like Bure, Bizona, and many others, have been engulphed by
earthquakes."

In another place, this learned geographer, in alluding to the tradition
that Sicily had been separated by a convulsion from Italy, remarks, that
at present the land near the sea in those parts was rarely shaken by
earthquakes, since there were now open orifices whereby fire and ignited
matters, and waters escape; but formerly, when the volcanoes of Etna,
the Lipari Islands, Ischia, and others, were closed up, the imprisoned
fire and wind might have produced far more vehement movements.[29] The
doctrine, therefore, that volcanoes are safety-valves, and that the
subterranean convulsions are probably most violent when first the
volcanic energy shifts itself to a new quarter, is not modern.

We learn from a passage in Strabo,[30] that