We have already seen that most of the chief towns of Queensland lie on or near the coast, and that there are many harbours, often protected by islands. The towns are there because the country was developed for the most part from the sea, and in fact settlement inland is still confined to the neighbourhood of the railways running up-country from these coast towns. The harbours are there because the land has sunk and the coast is partly drowned, giving deep sea-inlets, often where the sea has flooded some old river valley. As the coast gradually sank, the coral builders were at work, piling up their reefs in the warm shallows. So we get the Great Barrier Reef, stretching for fifteen hundred miles along 59 the coast and leaving a calm though rather dangerous channel between itself and the mainland. The reef is really a series of reefs, resting on a platform of older rock, and pierced with numerous openings, especially where the larger rivers enter the sea; for the coral will not grow in fresh water. In the neighbourhood of this reef, especially in the north, towards Thursday Island, fishing for pearl oysters is largely carried on. The oysters are valuable for the mother of pearl, rather than for the pearls themselves, which are very small. Here, too, is found in great quantity the trepang, or sea slug as it is sometimes 60 called, looking like millions of brown cucumbers crawling over the reef. Here is a portrait of one kind of the trepang; it does 61 not seem very appetising, but the Chinese consider it a great delicacy. Nearly the whole of the trepang gathered in this region is exported to China, after being first cooked and dried. The rest is eaten by the natives, as it does not appeal to European tastes.