These small islands are divided into two classes: the mountainous or volcanic and the coral or flat islands, also called "atolls", from a word in a South Indian language. Atolls, or coral islands, in the Pacific and Indian Oceans are made by the coral animals (a polyp or minute jellyfish-like creature living in communities like the cells of a human body), building upwards towards the surface of the sea the lime structures in which they exist. They start, of course, from some reef or submerged volcanic cone which is not far below the surface of the water. The limestone rock which they make out of secretions from the sea water gradually forms an irregular circle of narrow islets, with a lagoon in the centre and one or more deep openings to the sea. Vegetation--always coconuts--slowly covers these coral islands, and at last they become habitable by man. If the land beneath them is rising above the waves, the coral fossilizes into coralline rock and the living coral animals go on building and extending farther out to sea. If the land is sinking faster than the coral animal can build, then by degrees the once pleasant islets become a jumble of barren and crumbling rocks, only haunted by sea birds (but sometimes valuable for their guano), and finally a dangerous reef or shoal. Of the smaller Pacific islands that are not mere coral reefs and atolls the Hawaii group, the Marquezas, Tahiti, Easter Island, Pitcairn, the Samoa, Tonga, and Fiji groups, the New Hebrides, most of the Ladrones, and one or two of the Caroline Islands are _volcanic_, high, and of recent formation, even though like the New Hebrides and the Marquezas they may rest on submerged extensions of sunken lands. The Solomon Islands are also mainly volcanic, but with the Bismarck archipelago practically form an extension of New Guinea; while New Caledonia is a very old fragment of unsubmerged land, and so is tiny Norfolk Island between New Caledonia and New Zealand. The southern island of New Zealand is, like New Caledonia, a portion of an ancient continent which has never been submerged; and the north island of that dominion is a very volcanic region.