Coral Reefs [8]
have not thought it worth while to introduce all those small and very numerous reefs, which occur within the lagoons of most atolls and within the lagoon-channels of most barrier-reefs, and which stand either isolated, or are attached to the shores of the reef or land. At Peros Banhos none of the lagoon-reefs rise to the surface of the water; a few of them have been introduced, and are marked by plain dotted circles. A few of the deepest soundings are laid down within each reef; they are in fathoms, of six English feet.
Figure 1.--VANIKORO, situated in the western part of the South Pacific; taken from the survey by Captain D'Urville in the "Astrolabe;" the soundings on the southern side of the island, namely, from thirty to forty fathoms, are given from the voyage of the Chev. Dillon; the other soundings are laid down from the survey by D'Urville; height of the summit of the island is 3,032 feet. The principal small detached reefs within the lagoon-channel have in this instance been represented. The southern shore of the island is narrowly fringed by a reef: if the engraver had carried this reef entirely round both islands, this figure would have served (by leaving out in imagination the barrier-reef) as a good specimen of an abruptly-sided island, surrounded by a reef of the fringing class.
Figure 2.--HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the "Atlas of the Voyage of the 'Astrolabe,'" compiled from the surveys of Captains Duperrey and D'Urville; the depth of the immense lagoon-like space within the reef is not known.
Figure 3.--RAIATEA, in the Society Archipelago; from the map given in the quarto edition of "Cook's First Voyage;" it is probably not accurate.
Figure 4.--BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago, from the survey by Captain Beechey, R.N.; the lagoon is choked up with reefs, but the average greatest depth of about twenty fathoms, is given from the published account of the voyage.
Figure 5.--BOLABOLA, in the Society Archipelago, from the survey of Captain Duperrey in the "Coquille:" the soundings in this and the following figures have been altered from French feet to English fathoms; height of highest point of the island 4,026 feet.
Figure 6.--MAURUA, in the Society Archipelago; from the survey by Captain Duperrey in the "Coquille:" height of land about eight hundred feet.
Figure 7.--POUYNIPETE, or SENIAVINE, in the Caroline Archipelago; from the survey by Admiral Lutke.
Figure 8.--GAMBIER ISLANDS, in the southern part of the Low Archipelago; from the survey by Captain Beechey; height of highest island, 1,246 feet; the islands are surrounded by extensive and irregular reefs; the reef on the southern side is submerged.
Figure 9.--PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell; not nearly all the small submerged reefs in the lagoon are represented; the annular reef on the southern side is submerged.
Figure 10.--KEELING, or COCOS ATOLL (or lagoon-island), in the Indian Ocean; from the survey by Captain Fitzroy; the lagoon south of the dotted line is very shallow, and is left almost bare at low water; the part north of the line is choked up with irregular reefs. The annular reef on the north-west side is broken, and blends into a shoal sandbank, on which the sea breaks.
CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.
SECTION 1.I.--KEELING ATOLL.
Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.-- Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae subsisting on corals.--Changes in the condition of the reefs and islets.-- Probable subsidence of the atoll.--Future state of the lagoon.
(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)
A.--Level of the sea at low water: where the letter A is placed, the depth is twenty-five fathoms, and the distance rather more than one hundred and fifty yards from the edge of the reef.
B.--Outer edge of that flat part of the reef, which dries at
Figure 1.--VANIKORO, situated in the western part of the South Pacific; taken from the survey by Captain D'Urville in the "Astrolabe;" the soundings on the southern side of the island, namely, from thirty to forty fathoms, are given from the voyage of the Chev. Dillon; the other soundings are laid down from the survey by D'Urville; height of the summit of the island is 3,032 feet. The principal small detached reefs within the lagoon-channel have in this instance been represented. The southern shore of the island is narrowly fringed by a reef: if the engraver had carried this reef entirely round both islands, this figure would have served (by leaving out in imagination the barrier-reef) as a good specimen of an abruptly-sided island, surrounded by a reef of the fringing class.
Figure 2.--HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the "Atlas of the Voyage of the 'Astrolabe,'" compiled from the surveys of Captains Duperrey and D'Urville; the depth of the immense lagoon-like space within the reef is not known.
Figure 3.--RAIATEA, in the Society Archipelago; from the map given in the quarto edition of "Cook's First Voyage;" it is probably not accurate.
Figure 4.--BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago, from the survey by Captain Beechey, R.N.; the lagoon is choked up with reefs, but the average greatest depth of about twenty fathoms, is given from the published account of the voyage.
Figure 5.--BOLABOLA, in the Society Archipelago, from the survey of Captain Duperrey in the "Coquille:" the soundings in this and the following figures have been altered from French feet to English fathoms; height of highest point of the island 4,026 feet.
Figure 6.--MAURUA, in the Society Archipelago; from the survey by Captain Duperrey in the "Coquille:" height of land about eight hundred feet.
Figure 7.--POUYNIPETE, or SENIAVINE, in the Caroline Archipelago; from the survey by Admiral Lutke.
Figure 8.--GAMBIER ISLANDS, in the southern part of the Low Archipelago; from the survey by Captain Beechey; height of highest island, 1,246 feet; the islands are surrounded by extensive and irregular reefs; the reef on the southern side is submerged.
Figure 9.--PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell; not nearly all the small submerged reefs in the lagoon are represented; the annular reef on the southern side is submerged.
Figure 10.--KEELING, or COCOS ATOLL (or lagoon-island), in the Indian Ocean; from the survey by Captain Fitzroy; the lagoon south of the dotted line is very shallow, and is left almost bare at low water; the part north of the line is choked up with irregular reefs. The annular reef on the north-west side is broken, and blends into a shoal sandbank, on which the sea breaks.
CHAPTER I.--ATOLLS OR LAGOON-ISLANDS.
SECTION 1.I.--KEELING ATOLL.
Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.-- Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae subsisting on corals.--Changes in the condition of the reefs and islets.-- Probable subsidence of the atoll.--Future state of the lagoon.
(PLATE: UNTITLED WOODCUT, VERTICAL SECTION THROUGH KEELING ATOLL.)
A.--Level of the sea at low water: where the letter A is placed, the depth is twenty-five fathoms, and the distance rather more than one hundred and fifty yards from the edge of the reef.
B.--Outer edge of that flat part of the reef, which dries at