w abuts against it,
was formed. The inclination, strike, and fractures of the calcareous and
argillaceous strata of St. Elias have no relation to the great cone,
but, according to M. Bory St. Vincent, have the same direction as those
of the other isles of the Grecian Archipelago, namely, from N. N. W. to
S. S. E. Each of the three islands, Thera, Therasia, and Aspronisi, is
capped by an enormous mass of white tufaceous conglomerate, from forty
to fifty feet thick, beneath which are beds of trachytic lava and tuff,
having a gentle inclination of only 3 degrees or 4 degrees. Each bed is
usually very narrow and discontinuous, the successive layers being
moulded or dove-tailed, as M. Virlet expresses it, into the inequalities
of the previously existing surface, on which showers of cinders or
streams of melted matter have been poured. Nothing, therefore, seems
more evident than that we have in Santorin the basal remains of a great
ruined cone, or flattened dome; and the absence of dikes in the cliffs
surrounding the gulf would indicate that the eruptions took place
originally, as they have done in the last two thousand years, not near
the margin but in the centre of the space now occupied by the gulf. The
central portions of the dome have since been removed by engulfment, or
denudation, or by both these causes.

An important fact is adduced by M. Virlet, to show that the gentle dip
of the lava-streams in the three outer islands towards all points of the
compass, away from the centre of the gulf, has not been due to the
upheaval of horizontal beds, as conjectured by Von Buch, who had not
visited Santorin.[613] The French geologist found that the vesicles or
pores of the trachytic masses were lengthened out in the several
directions in which they would have flowed if they had descended from
the axis of a cone once occupying the centre of the crater. For it is
well known that the bubbles of confined gas in a fluid in motion assume
an oval form, and the direction of their longer axis coincides always
with that of the stream.

On a review, therefore, of all the facts now brought to light respecting
Santorin, I attribute the moderate slope of the beds in Thera and the
other external islands to their having originally descended the inclined
flanks of a large volcanic cone, the principal orifice or vents of
eruption having been always situated where they are now, in or near the
centre of the space occupied by the gulf or crater--in