and composed of strata dipping outwards, once extended round a considerable
portion of the circumference of the island. The ring appears to have been
oval and of vast size; its shorter axis, measured across from the inner
sides of the mountains near Port Louis and those near Grand Port, being no
less than thirteen geographical miles in length. M. Bailly boldly supposes
that this enormous gulf, which has since been filled up to a great extent
by streams of modern lava, was formed by the sinking in of the whole upper
part of one great volcano.

It is singular in how many respects those portions of St. Jago and of
Mauritius which I visited agree in their geological history. At both
islands, mountains of similar external form, stratification, and (at least
in their upper beds) composition, follow in a curved chain the coast-line.
These mountains in each case appear originally to have formed parts of one
continuous mass. The basaltic strata of which they are composed, from their
compact and crystalline structure, seem, when contrasted with the
neighbouring basaltic streams of subaerial formation, to have flowed
beneath the pressure of the sea, and to have been subsequently elevated. We
may suppose that the wide breaches between the mountains were in both cases
worn by the waves, during their gradual elevation--of which process, within
recent times, there is abundant evidence on the coast-land of both islands.
At both, vast streams of more recent basaltic lavas have flowed from the
interior of the island, round and between the ancient basaltic hills; at
both, moreover, recent cones of eruption are scattered around the
circumference of the island; but at neither have eruptions taken place
within the period of history. As remarked in the last chapter, it is
probable that these ancient basaltic mountains, which resemble (at least in
many respects) the basal and disturbed remnants of two gigantic volcanoes,
owe their present form, structure, and position, to the action of similar
causes.