e rendered visible above the water, the
central craters of eruption; and between these and the inner cliff of
Thera and Therasia is a ring of deep water, descending to a depth of
over 200 fathoms. So that, were these islands raised out of the sea, we
should have presented to our view a magnificent circular crater about
six miles in diameter, bounded by nearly vertical walls of rock from
1000 to 1500 feet in height, and ruptured at one point, from the centre
of which would rise two volcanic cones--namely, the Kaimenis--one with a
double crater, still foci of eruption, and from time to time bursting
forth in paroxysms of volcanic energy, of which those of 1650, 1707, and
1866 were the most violent and destructive.[1] Of this last I give a
bird's-eye view (Fig. 14).

The only rock of non-volcanic origin in these islands consists of
granular limestone and clay slate forming the ridge of Mount St. Elias,
which rises to a height of 1887 feet at the south-eastern side of the
island of Thera, crossing the island from its outer margin nearly to the
interior cliff, so that the volcanic materials have been piled up along
its sides. The rocks of St. Elias are much more ancient than any of the
volcanic materials around; and, as Bory St. Vincent has shown, have been
subjected to the same flexures, dip and strike, as those sedimentary
rocks which go to form the non-volcanic islands of the Grecian
archipelago.

[Illustration: Fig. 14.--Bird's-eye View of the Gulf of Santorin during
the volcanic eruption of February 1866.--(After Lyell.)]

[Illustration: _Ground Plan of Rocca Monfina_
Fig. 15.--Rocca Monfina, in Southern Italy, showing a crater-ring of
trachytic tuffs, from the midst of which, according to Judd, an andesite
lava-cone has been built up. Compare with the Santorin Group.]

(_b._) _Origin of the Santorin Group._--In reference to the origin of
the Santorin group, Lyell regards it as a remnant of a great volcanic
mountain which possessed a focus of eruption rising in the position of
the present foci, but afterwards partially destroyed and the whole
submerged to a depth of over 1000 feet. But another explanation is open
to us, and one not inconsistent with what we now know of the physical
changes to which the Mediterranean has been subjected since early
Tertiary times. To my mind it is difficult to conceive how such a
volcanic mountain as that of Santorin could have been formed under
water; while, on the other hand, its ph