is manner the original crater may have been slowly widened and
deepened, after which the whole archipelago may have been partially
submerged to its present depth.

That such oscillations of level may in the course of ages have taken
place, will be the more readily admitted when we state that part of
Thera has actually sunk down in modern times, as, for example, during
the great earthquake before alluded to, which happened in 1650. The
subsidence alluded to is proved not only by tradition, but by the fact
that a road which formerly led between two places on the east coast of
Thera is now twelve fathoms under water.

MM. Boblaye and Virlet mention,[611] that the waves are constantly
undermining and encroaching on the cliffs of Therasia and Aspronisi, and
shoals or submarine ledges were found, during the late survey, to occur
round a great part of these islands, attesting the recent progress of
denudation. M. Virlet also remarks, in regard to the separation of the
three islands forming the walls of the crater, that the channels between
them are all to the W. and N. W., the quarter most exposed to the waves
and currents.

Mr. Darwin, in his work on volcanic islands, has shown that in the
Mauritius and in Santiago, there is an external circle of basaltic rocks
of vast diameter, in the interior of which more modern eruptions have
taken place, the older rocks dipping away from the central space in
every direction, as in the outer islands of Santorin. He refers the
numerous breaches, some of them very wide in the external ramparts of
those islands, to the denuding action of the sea. Every geologist,
therefore, will be prepared to call in the aid of the same powerful
cause, to account for the removal of a large part of the rocks which
must once have occupied the interior space, in the same manner as they
attribute the abstraction of matter from elliptical "valleys of
elevation," such as those of Woolhope and the Wealden in England, to the
waves and currents of the sea.

Thera, Therasia, and Aspronisi are all composed of volcanic matter,
except the southern part of Thera, where Mount St. Elias rises to three
times the height of the loftiest of the igneous rocks, reaching an
elevation of 1887 feet above the sea.[612] This mountain is formed of
granular limestone and argillaceous schist, and must have been
originally a submarine eminence in the bed of the Mediterranean, before
the volcanic cone, one side of the base of which no