from some cause not ascertained the volcanic
districts of the Mediterranean and its shores are confined to the north
side of that great inland sea; so that as regards vulcanicity the
African coast presents a striking contrast to that of the opposite side.
If we draw a line from the shores of the Levant to the Straits of
Gibraltar, by Candia, Malta, and to the south of Pantelleria and
Sardinia, we shall find that the volcanic islands and districts of the
mainland lie to the north of it.[3] This has doubtless some connection
with the internal geological structure. The immunity of the Libyan
desert from volcanic irruptions is in keeping with the remarkably
undisturbed condition of the Secondary strata, which seldom depart much
from the horizontal position; while the igneous rocks of the Atlas
mountains are probably of great geological antiquity. On the other hand,
the Secondary and Tertiary formations of the northern shores and islands
of the Mediterranean are generally characterised by the highly-inclined,
flexured, and folded position of the strata. Hence we may suppose that
the crust over the region lying to the north of the volcanic line, owing
to its broken and ruptured condition, was less able to resist the
pressure of the internal forces of eruption than that lying to the south
of it; and that, in consequence, vents and fissures of eruption were
established over the former of these regions, while they are absent in
the latter.

[1] Fuller details will be found in Daubeny's _Volcanoes_, chap. xviii.,
and Lyell's _Principles of Geology_, vol. ii. p. 65 (edition 1872). The
bird's-eye view is taken from this latter work by kind permission of the
publisher, Mr. J. Murray, as also the accompanying Ideal Section, Fig.
13.

[2] Wallace, _Geographical Distribution of Animals_ (1876). The author's
_Sketch of Geological History_, p. 130 (Deacon & Co., 1887).

[3] The _volcanic area_ lying to the north of this line will include
Sardinia, Sicily, Pantelleria, the Grecian Archipelago, Asia Minor, and
Syria; the _non-volcanic area_ lying to the south of this line will
include the African coast, Malta, Isles of Crete and Cyprus. The Isle of
Pantelleria is apparently just on the line, which, continued eastward,
probably follows the north coast of Cyprus, parallel to the strike of
the strata and of the central axis of that island.--See "Carte
Geologique de l'ile de Chypre, par MM. Albert Gaudry et Amedee Damour"
(1860).