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Updated: June 22, 2025
The evidence from a single atoll or a single encircling barrier-reef, must be received with some caution, for the former may possibly be based upon a submerged crater or bank, and the latter on a submerged margin of sediment, or of worn-down rock.
Reaching Van Diemen's Land in August, and Australia soon after, they sailed from Port Stephens December 19, to commence their survey. After an examination of the Capricorn Group, they commenced the survey of the northern part of the great barrier-reef, up to the Murray Islands. In the next year, they erected a beacon on Raines Islet to mark the entrance of a good passage through the reef.
In all these works their author, while maintaining an independent judgment on certain matters of detail, warmly defends the views of Darwin on all points essential to the theory. Another able exponent and illustrator of the theory of coral-reefs was found in Professor J.B. Jukes, who accompanied H.M.S. "Fly", as naturalist, during the survey of the Great Barrier-Reef in the years 1842 to 1846.
Let us take our new encircling barrier-reef, of which the section is now represented by unbroken lines, and which, as I have said, is a real section through Bolabola, and let it go on subsiding.
The instant this takes place, a perfect atoll is formed: I have said, remove the high land from within an encircling barrier-reef, and an atoll is left, and the land has been removed.
The origin of this latter reef, which might have been mistaken for a barrier-reef, has already been attempted to be explained, when giving the proofs of the recent elevation of this island. In Cook's charts the little outlying island also of EOAIGEE, is represented as fringed; coloured red.
The entire ANNULAR REEF, which when surrounding an open expanse of water, forms an "atoll," and when surrounding one or more high islands, forms an encircling "barrier-reef," has a nearly uniform structure.
The Great Comoro Island, which probably contains a volcano, is only twenty miles distant from the barrier-reef of Mohila; Ambil volcano, in the Philippines, is distant only a little more than sixty miles from the atoll-formed Appoo reef: and there are two other volcanoes in the map within ninety miles of circles coloured blue.
An atoll differs from an encircling barrier-reef only in the absence of land within its central expanse; and a barrier-reef differs from a fringing-reef, in being placed at a much greater distance from the land with reference to the probable inclination of its submarine foundation, and in the presence of a deep-water lagoon-like space or moat within the reef.
If this reef had been more extensive, it must have been classed as a barrier-reef; but as the line of coast falls inwards here, a submarine bank perhaps extends parallel to the shore, which has offered a foundation for the growth of the coral; I have left this part uncoloured.
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