been superimposed. In St. Helena we
seem, according to Daubeny, to have the mere wreck of one great crater,
no one stream of lava being traceable to its source, while dykes of lava
are scattered in profusion throughout the whole substance of the
basaltic masses which compose the island. Tristan da Cunha, in the
centre of the South Atlantic, rises abruptly from a depth of 12,150
feet, at a distance of 1500 miles from any land; and one of its summits
reaches an elevation of 7000 feet, being a truncated cone composed of
alternating strata of tuff and augitic lava, surrounding a crater in
which is a lake of pure water. The volcano is extinct or dormant.

Were the waters of the ocean to be drawn off, these volcanic islands
would appear like stupendous conical mountains, far loftier, and with
sides more precipitous, than any to be found on our continental lands,
all of which rise from platforms of considerable elevation. The enormous
pressure of the water on their sides enables these mid-oceanic islands
to stand with slopes varying from the perpendicular to a smaller extent
than if they were sub-aerial; and it is on this account that we find them
rising with such extraordinary abruptness from the "vasty deep."

(_d._) _Volcanic Islands of the Pacific._--The volcanic islands of this
great ocean are scattered over a wide tract on both sides of the
equator. Those to the north of this line include the Sandwich Islands,
the Mariana or Ladrone Islands, South Island, and Bonin Sima; south of
the equator, the Galapagos, New Britain, Salomon, Santa Cruz, New
Hebrides, the Friendly and Society Isles. While the coral reefs and
islands of the Pacific may be recognised by their slight elevation
above the surface of the waters, those of volcanic origin and containing
active or extinct craters of eruption generally rise into lofty
elevations, so that the two kinds are called the _Low_ Islands and
_High_ Islands respectively. Amongst the group are trachytic domes such
as the Mountain of Tobreonu in the Society Islands, rising to a height
probably not inferior to that of Etna, with extremely steep sides, and
holding a lake on its summit.[5] The linear arrangement of some of the
volcanic islands of the Pacific is illustrated by those of the Tonga, or
Friendly, Group, lying to the north of New Zealand. They consist of
three divisions--(1) the volcanic; (2) those formed of stratified
volcanic tuff, sometimes entirely or partially covered by cora