NARRATIVE OF THE CRUISE. 
781 
incrustations, and at first sight presenting a fallacious resemblance to a Coral such as 
Millepora. 
The margins of the reefs inside the lagoons presented quite a different appearance from 
those on the seaward face outside. Here they were fringed with living coral, sloped 
.downwards and outwards for a few feet, and then plunged at once to depths of 10 and 
16 fathoms. Many of them were overhanging, and in one place a large ledge seemed 
recently to have fallen away, and to have rendered the water shallower alongside the 
reef. These overhanging or mushroom-shaped reefs inside lagoons or lagoon-channels 
have been frequently described by Jukes and others. The deposit in the lagoons was 
in some places a coral sand and in others a volcanic mud. The reefs at Tahiti have been 
described both by Darwin and Dana as examples of barrier reefs, but for miles the reefs 
have quite a fringing character ; in other places there is only a small channel across 
which the natives wade to the edge of the barrier, or there is a boat passage ; in other 
places, as at Papiete, there is a ship channel, or a commodious harbour, while there are 
some portions of the coast where no reefs exist. 
The island of Tahiti is surrounded by a belt of fertile land from 3 to 4 miles wide, 
and in some places the alluvium rests on portions of the shore reef in such a way as to 
indicate that the island had been recently elevated. 
According to Mr. Murray the observations of the reefs at Tahiti support the view that 
the reefs have been built from the shore seawards, and that the lagoons have been, and 
are still being, formed by the removal of the inner and dead portions of the coral reef 
by the solvent action of sea water. The islands in the harbour and lagoons are regarded 
as portions of the reef which have been left standing, but will ultimately be removed, 
and in confirmation of this it is pointed out that on the inner part of the reef there are 
large and massive specimens of the coral which are now dead, but which probably 
flourished at the time when the outer edge of the reef was at the position in which they 
are now found. The steep slope which is found on the outer edge of the reef, between 
the depths of 35 and 200 fathoms, is believed to be formed by huge masses and heads of 
coral which have been torn away from the ledge between the edge of the reef and 35 
fathoms during storms, or by overhanging masses which have fallen by their own weight. 
In this way a talus has been formed on which the corals living down to 35 fathoms have 
found a foundation on which to build further seawards, for this seaward slope is the great 
growing surface of the reef. The food supply for the masses of living coral on the outer 
slope of the reef is brought by the oceanic currents sweeping past the islands, a fact in 
relation with the more vigorous growth of the reef on the windward sides. It is 
maintained by Mr. Murray 1 that the whole of the phenomena of the Tahiti reefs may be 
fully explained by reference to the processes at present in action, and without calling 
1 On the Structure and Origin of Coral Eeefs and Islands, by John Murray, Proc. Roy. Soc. Edin., vol. x. 
pp. 505-518, 1880. 
