BARRIER REEF. The Great Barrier Reef, the largest con tinuous mass of coral in the world, lies off the north-east coast of Australia and extends virtually from New Guinea south of the Fly River delta (Bramble cay : 9° 15' S., 153° 20' E.) to about Sandy cape (24° 30' S., 150° 20' E.), a distance of I,Ioo nautical (1,260 statute) miles. On the north its termination is probably due to the fresh muddy waters of the Fly river, on the south to the increasing coolness of the waters (68° F) . It follows fairly closely the edge of the continental shelf—which here apparently represents a submerged (Pleistocene?) coastline—at about 5o fathoms. Beyond it steep submarine slopes lead down to depths of over 100 fathoms : behind it are depths of 10-25 fathoms, and towards the south of 40-60 fathoms, for the most part over a sandy bottom. In the north (Cape York peninsula) the reef keeps within 20 m. of the coast and here it is composed of sections each a few miles long separated by gaps a mile wide. Farther south it draws away to distances of 12o-15o m. from the coast and also gradually breaks up into small scattered clusters. Throughout its length it varies greatly in width. Between the reef and the mainland are numerous islands, those adjoining the coast being often lofty, rugged and densely wooded fragments of the hill ranges behind. Here the muddiness or freshness of the water (Burdekin, Fitzroy, etc. rivers) operates against coral growth, and the small flat coralline islets (atolls) are in clearer waters farther out. Much discussion has centered around the formation and precise significance of the Barrier Reef. Of the two main hypotheses—the "glacial-control" hypothesis which postulates a rise in the sea-level due to melting ice, and the subsidence theory —the latter has gained most support. Trial borings (Funafuti, Ellice islands in 1903; Oyster cay, 22 m. N.E. of Cairns, 1926) have proved inconclusive, but the numerous evidences of sub mergence and "drowning" of coastal features in this region (cf. the recent and perhaps still continuing subsidence of the sea-floor north of Sandy cape) support other indications that the coral has grown upon or near the sinking rim of the continent. The outer (eastern) face of the coral is the most active and vigorously grow ing part, rearing nearly perpendicular walls from depths of 20-25 fathoms to a little below low-tide level. Above this the coral is dead and is smashed by storms into blocks which are piled up (cf. the "negro-heads," isolated blocks standing up above high-water level), ground into sand, re-compacted and ultimately form steep sided banks and low flat-topped islets. Behind the permanently submerged growing "front," with its well-known, but almost un believable, riot of colour and animal life, and its tidally sub merged or above-water bank, is a zone of feebler and more irregu lar growth studded with numerous basins and holes. This zone sinks gradually westwards and from it rise mushroom-topped pin nacles of growing coral. Each segment of reef had its "plan" determined mainly by prevailing wind and wave direction, being usually curved in at either end and tending to form a closed oval, which then often fills up and becomes a "cay." The coral islets are colonized fairly rapidly by a select assemblage of salt-tolerant plants whose seeds are borne by birds, winds or waves—man groves, casuarinas, pandanus palms and other creeping and flowering types. (Coconut palms are never "self-sown" here.) The rich and varied animal life includes many gaily-coloured fish, sea-urchins with poisonous spines, giant clams, star-fish, etc. Pearl oysters, once very plentiful, are still systematically fished, and these, with the trochus shell (resembling a large periwinkle) yield considerable quantities of the mother-of-pearl of trade. Another important economic product is the beche-de-mer (tre pang) or sea-cucumber, which is boiled, dried and exported in great bulk to China. Dugong, turtles and sponges (the "hard" variety) occur but are not as yet much exploited. The interior passage-way (the "Grand Canal") offers a fairway for vessels trad ing up this coast—sheltered and calm, while outside the breakers form a long line of surf. But the numerous rocks and reefs, the hurricanes which occur as far south as lat. 16° S., the tides which, pent in the narrows, rise in places 3o ft., and in the north the nar rowness and complexity of the passageways make this also one of the most dangerous coasts of Australia. Numerous Barrier Reef problems have been systematically studied in Australia and a scientific expedition organised by the British Association went out in 1928 to undertake further investigations.