they may procure a supply of good water, than which nothing is more
, important to the navigation of these seas; as that indispensable article
is not found to exist in a pure state anywhere between Otaheite and
the coast of Chili, a distance of 4000 miles, Pitcairn Island excepted,
where the difficulty of getting it off has already been mentioned. It is
also presumed, that the position of the islands having been ascertained,
the peaks of Mount Duff, which are high and distinguishable at a great
distance, will serve as a guide to the labyrinth of coral islands which
the navigator, after passing this groupe, has to thread on his way to the
westward.
This groupe was discovered by the ship Duff, on a missionary voyage,
in 1797, and named by Mr. Wilson, her commander, after Admiral Lord
Gambier. It consists of five large islands and several small ones, all
situated in a lagoon formed by a reef of coral. The largest is about
six miles in length, and rises into two peaks, elevated 1248 feet above
the level of the sea. These peaks, which were called after the Duff,
are in the form of wedges, very conspicuous at a distance, and may be
seen fourteen or fifteen leagues. All the islands are steep and rugged,
particularly Marsh Island, which at a distance resembles a ship. The
external form of these islands at once conveys an impression of their
volcanic origin; and, on examination, they all appeared to have been
subjected to the action of great heat.
“ The general basis of the rocks is a porous basaltic lava, in one
place passing into a tuffacious slate; in another, into the solid and
angular column of compact basalt, containing the imbedded minerals
which characterise this formation, and bearing a close resemblance in
this particular to the basaltic formation of the county of Antrim in
Ireland. There is, however, less of the basalt and more of the porous.
The zealites, soapstone, chalcedony, olivine, and calcareous spar, are
formed in, and connect the relationship of these distant formations;
whilst the different-coloured jaspers are peculiar to these islands. There
is also another obvious distinctive feature produced by the numerous
dykes of a formation differing in composition and texture, and marked by
a defined line. They are generally more prominent than the common
rock; traversing a great many, if not all the islands, in a direction nearly
east and west; generally about eighteen inches wide, nearly perpen- C lEA l
dicular to the horizon, or dipping to the southward. Their texture
is sometimes compact, sometimes vesicular, with few if any imbedded
J an .
1820.
minerals, excepting one on Marsh Island, which contained great quantities
of olivine. Upon a small island contiguous to this, the harder
dyke crosses the highest ridge, and divides on the eastern side into two
parts which continue down to the water’s edge*.”
Lieutenant Belcher, whose scientific attainments also enabled him
to appreciate what fell under his observation, noticed every where the
trap formation abounding in basaltic dykes also lying N . E. and S. W.,
and seldom deviating from the perpendicular; or if they did, it was to the
eastward. W e are indebted to him for specimens of zealite, carbonate ot
lime, calcareous spar, crystals, an alcime, olivine, jasper, and chalcedony;
and had our stay, and his other duties admitted, we should, no doubt,
have received from him a more detailed account of this interesting
groupe.
There are no appearances of pseudo-craters on any of the islands,
nor do they seem to have been very recently subjected to fire, being
clothed with verdure, and for the most part with trees. Conspicuously
opposed to these lofty rugged formations, raised by the agency of fire,
is a series of low islands, derived from the opposite element, and owing
their construction to myriads of minute lithopliytes, endowed with an
instinct that enables them to separate the necessary calcareous matter
from the ocean, and with such minute particles to rear a splendid structure
many leagues in circumference. A great wall of this kind, if we
may use the expression, already surrounds the islands, and, by the unremitting
labour of these submarine animals, is fast approaching the
surface of the water in all its parts. On the N. E. side, it already bears
a fertile soil beyond the reach of the sea, sustains trees and other subjects
of the vegetable kingdom, and affords even an habitation to
man.
In the opposite direction it dips from thirty to forty feet beneath
the surface, as if purposely to afford access to shipping to the lagoon
* M r. Collie’s Journa l.
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