Western Pacific Arcs. I. New Guinea, New Caledonia.—This arc may be traced through the Louisiade islands, a prolongation of south-east New Guinea, New Caledonia, with the Loyalty islands near by, and Norfolk island, perhaps to north-west New Zealand. New Caledonia has a band of Archaean and Palaeozoic rocks running almost the whole length of the island from north-west to south-east, while what are probably Eocene beds fringe the south west coast. Much of the old rock is slate or schist with serpen tine, granitic and basic volcanic intrusions. The slates are inter bedded with limestones containing fossil brachiopods, referred to the Trias. There are evidences of submergence and a wide lagoon barrier reef. There is a general geological resemblance between New Caledonia and New Zealand. The main mineral veins are of nickel (garnierite) in the eastern peridotite dikes. The Eocene rocks disappear in places beneath the older ones, thus demonstrating overfolding, the direction of which may have been south-westwards. There is thus no doubt that here is a remnant of a folded mountain chain. Some of the fossils found in New Caledonia are northern forms.
Western Pacific Arcs. II. New Britain Archipelago. New Hebrides.—This is traceable especially through New Ireland, the Solomons and Santa Cruz to the New Hebrides.
New Ireland possesses porphyries, diorite, some dark calcareous rock and some grey rather crystalline limestone reaching nearly 2,50o feet above the sea. The south of New Ireland is volcanic but, to the north, sedimentary rocks rest on an ancient mass which is scarped towards the south-west and probably represents the most easterly of the New Guinea folds. The other islands of the New Britain group, as well as the Admiralty islands, are volcanic, with recent coral. The features of New Ireland, including the grey limestone, are continued along the chain to the New Hebrides, and the limestone, containing Rhynchonella sp., is thought to be Mesozoic. Several active volcanoes occur at what appear to be breaks in the chain which may be considered a fragment of a cordillera. Former statements as to ancient rocks in the New Hebrides have not been confirmed.
Western Pacific Arcs. III. Caroline Islands, Marshall Islands, Fiji.—The general direction of the chain from the Marshall islands to Fiji north-north-west to south-south-east and the rela tion of the Fijis to the others may not be simple because of the in terference of the Samoan line discussed below. The Fiji islands are mainly volcanic including many tuffs interstratified with tertiary limestone and resting on limestone older but still tertiary. In the south of Viti Levu is a high granitic plateau which is said to be a horst. Vanua Levu has hot springs. Fiji yields malachite, antimony, graphite, gold in small quantities, and specular iron sand. W. M. Davis has made a special study of the coral reefs of Fiji and thinks that they are formed by the method inferred by Darwin. He also notes that on the south-west side of Viti Levu and on the south side of Vanua Levu there are raised reefs and fringing reefs but no barrier reefs off shore ; while on the north and north west sides of the island there are off-shore barrier reefs but no raised reefs. The Ellice, Gilbert and Marshall islands north of
Fiji are volcanic with coral atolls. The politically-famous Nauru island (Pleasant island) lies south-west of the Marshall group. The Carolines, west of the Marshalls, include several small chains not quite parallel to the main direction of the whole arc ; it has been surmised that they are built upon a deeply foundered fold system.
Western Pacific Arcs. IV. New Zealand, Tonga.—The Ruahine chain of New Zealand is a scheme of fold mountains with a palae ozoic axis parallel to the east coast of North island, New Zealand. To the west of it lies a volcanic zone including the peaks of Ruapehu and Tongariro, Lake Taupo, Rotorua, and the bay of Plenty with the volcano of White island. The line is continued north along the Kcrmadec to the Tonga islands. The western side of the Tonga chain is highly volcanic ; Hongu-tonga and Hongu-hapai appear to be fragments of a single crater, and Toqua Kao, Late, Metis, Amargua and Falcon islands are also purely volcanic; the rock is basic. Tonumeia and the Nomuka group (Mango, Nomuka-iki, etc.) have coral limestone over submarine volcanic deposits of tuffs, penetrated by basic volcanic dikes. The Vavau group, Tongatabu and Hapai do not show a volcanic sub stratum. Metis and Falcon islands are usually submerged.
Samoa, Tokelau Is., Phoenix Is.—The fourth arc just discussed is bounded by a long sea-deep which however has its continuity broken just where certain Polynesian island-chains cut it north of the Tonga islands. The chief of these is the Samoan group but the Tokelau (Union) islands and apparently the Phoenix islands may be mentioned here.
The Samoan chain shows vulcanicity, oldest in the east where the craters have disappeared, less old in the centre where they form peaks, and obviously very recent in the west.
The Eastern Polynesian Chains, orientated west-north-west to east-south-east have as a foundation a plateau submerged beneath 700-800 fathoms of water, on the east side of the zone of great depths. The island-chains diminish and disappear south-eastwards and here the ocean becomes 2,000 fathoms deep and more. The Austral and Cook islands are the most south-westerly chain. The Society islands' (Tahiti) form the middle one and include some atolls but are mainly a basic volcanic group. Tahiti, itself ringed by strong cliffs with much evidence of submergence, has a dis continuous barrier reef off-shore. Here the submergence is older than in the Marquesas and the embayed valleys are filled with alluvium. Though all volcanic activity has now ceased, remains of plants and animals now living in the islands are found be neath some of the lava flows, so vulcanicity is recent. The north easterly chain is that of the Tuamotu or Low Archipelago, in which Makatea (Metis) reaches a height of some 23o feet. Most of the islands are atolls which Agassiz thought were built on denuded summits of Tertiary limestone, cut at about sea-level. The Manahiki islands farther north-west may be related to the Tuamotu.