82 AUSTEALASIA. 



polypi flourish best as a rule ou the outer riiu of the reefs, where they are exposed 

 to the fresh curreuts and wash of the tides, and here their buildings most rapidly 

 rise to hio-h-water level. Then their further growth above the surface and trans- 

 formation to islands or continental seaboards is the work of storms. Huge blocks 

 detached from the encircling reef are thrown together in rude heaps, and gradually 

 consolidated by fresh additions. Then the dry surface is weathered and prepared 

 for the reception of the seeds brought by wind and water. Here the seafowl build 

 their nests, the germs strike root, grasses and shrubs spring up on the new land 

 thus born of the tempest. 



The form and appearance of the upheaved coral structures differ greatly accord- 

 ino- to the regions where they have been constructed. The least noteworthy are 

 the barrier reefs which fringe the insular and continental shore lines, and which 

 rest on a foundation of shelving rocks. But in many places the reefs are not in 

 contact with the coasts around which they have grown up, but are developed at 

 some distance seawards, leaving here and there a navigable passage, or at least 

 a flooded channel between their inner edge and the mainland. Some of these 

 formations extend for hundreds, and in the case of the Great Barrier Reef of 

 Australia for over 1,000 miles along the coast. Others, such as the annular reef 

 of New Caledonia, completely encircle the island, which remains as a central 

 nucleus to the system. A slight upheaval would change to dry land the inter- 

 mediate space between the island and the ring, thus doubling or trebling the 

 extent of the raised surface. 



Lastly, there are thousands of systems which have no central nucleus, and 

 which consist of nothing but a perfect or fragmentary ring enclosing an inner 

 lagoon either still communicating with or separated from the sea and gradually 

 silting up with the accumulating sands and organic débris. Some of these lagoons 

 have even been transformed to freshwater basins by the slow action of the rains. 

 To all annular reefs has been extended the term atoll from those of the Mal- 

 dive Archipelago, the most regular and numerous group found in the whole 

 ocean. 



Every possible transitional form occurs between the barrier reef skirting the 

 mainland and the perfectly circular atoll lashed on its outer rim by the stormy 

 seas, and enclosing an inner lagoon of smooth water. Most of the forty thousand 

 rocks and islets in the Maldive Archipelago are so disposed as to form atolls 

 within atolls, that is to say, each fragment of a ring is itself a ring. 



The study of the coralline reefs led the illustrious Darwin to form some bold 

 generalisations on the slow oscillations of the terrestrial crust. Finding that the 

 barrier reefs and outer walls of the atolls rise in many places above deep waters, 

 he concludes that these rocks were entirely built by the same polypi who are still 

 piling up similar structures. But as they can work only in the surface waters 

 where the ceaseless ebb and flow brings them the materials of their edifices, the 

 great elevation of so many coralline rocks would seem to attest a gradual subsi- 

 dence of the marine level. The first colonies began their operations within about 

 120 feet of the surface ; but according as the structures rose the ground sank, and 



