GEOLOGY. 
165 
GAMBIER ISLANDS. 
The nine largest and most elevated of this group are of volcanic origin. There are 
also two small and low islands which are the production of corals. One of these has 
not yet given root to any vegetable. Corals abound to a great extent beneath the 
surface of the water, and are gradually rising in the whole space between the volcanic 
islands. They are also surrounding the group with a wall of circumvallation several 
yards in breadth. 
The general character of the rock composing these islands is a porous basaltic lava, 
in one place passing into argillaceous tufa, in another into solid and angular columns of 
compact basalt. 
The imbedded minerals are zeolites, soap-stone, chalcedony, olivine and calcareous 
spar, as in the north of Ireland; there are different coloured jaspers which are peculiar 
to these islands. They contain also numerous dykes, traversing the strata in a direc- 
tion generally from east to west, and differing from the strata they intersect in greater 
solidity and durability, and in containing a greater quantity of olivine ; they often pro- 
ject in walls from one to four feet high, and are from one to three feet wide. These 
veins or dykes are separated by a well-defined line from the general rock which they 
pass through ; some dip slightly to the south, others are perpendicular. They appear to 
traverse all the islands, occasionally bifurcating, as upon the eastern side of the small 
island next to Marsh Island. They are sometimes porous, and sometimes, when com- 
pact, contain a few particles of olivine; but they almost always, whatever be their indi- 
vidual structure, rise prominent from the surrounding rock. 
These islands preserve the general form of igneous productions in other parts of 
the world, rising out of the ocean by black perpendicular cliffs, and terminating at a 
considerable height in conical peaks. Two of them are particularly remarkable, being 
solitary, and when seen from a considerable distance at sea, with a thin cloud hovering 
over them, have the appearance of smoking volcanos. Small patches of productive soil 
have been formed at the bottom of the hills, by the rain washing down the disintegrated 
particles from the declivities, and the waves of the sea washing up sand and fragments 
of coral. These spaces (with a very few spots on the slopes) are the parts that produce 
vegetable food or timber, e. g. the cocoa-nut, bread-fruit tree, banana, the yappe, the 
sweet potato, the amai or miro, the cloth-tree, the doodoe, the hibiscus, rosa sinensis, 
and some others. 
The extensive basin within the vast wall of coral that encircles this group of islands 
is more or less filled up, mostly so along the shores, where the numerous crowds of coral 
animals have raised their labours to the surface at low water, or even up to half-tide. 
In the intervening space the bottom is very uneven, at one place twenty fathoms, at 
another only of three or four from the surface. The corals which most abound are the 
explanaria, astrea, madrepora cervicornis, meandrina, and pocillipora damicornis. 
The transparent limpidity of the water permits the eye clearly to see these different 
zoophytes at from five to seven fathoms’ depth, forming, as it were, a submarine shrub- 
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