Volume 14 Australasia - dana ward's homepage

32 AUSTRALASIA.polypi flourii-h best as a rule on the outer rim of tlie reefs, where they are exposedlo the fresh currents and wash of the tides, and here their buildings most rapidlyrise to high-water level.Then their further growth above the surface and transformationto islands or continental seaboards is the work of storms. Huge blocksdetached from the encircling reef are thrown together in rude heaps, and graduallyconsolidated by fi-e.sh additions. Then the dry surface is weathered and preparedfor the reception of the seeds brought by wind and water. Here the seafowl buildtheir nests, the gei'ras strike root, grasses and shrubs spring up on the new landthus born of the tempest.The form and appearance of the upheaved coral structures differ greatly accordingto the regions where they have been Constructed. The least noteworthy arethe barrier reefs which fringe the insular and continental shore lines, and whichrest on a foundation of shelving rocks. But in many places the reefs are not incontact with the coasts around which they have grown up, but are developed atsome distance seawards, leaving here and there a navigable passage, or at leasta flooded channel between their inner edge and the mainland. Some of theseformations extend for hundreds, and in the case of the Great Barrier Reef ofAustralia for over 1,000 miles along the coast. Others, such as the annular reefof New Caledonia, completely encircle the island, which remains as a centralnucleus to the sj'stem. A slight upheaval would change to dry land the intermediatespace between the island and the ring, thus doiibling or trebling theextent of the raised surface.Lastly, there are thousands of systems which have no central nucleus, andwhich consist of nothing but a perfect or fragmentary ring enclosing an innerlagoon either still communicatiug with or separated from the sea and graduallysilting up with the accumulating sands and organic debris. Some of these lagoonshave even been transformed to freshwater basins by the slow action of the rains.To all annular reefs has been extended the term afo/l from those of the MaldiveArchipelago, the most regular and numerous group found in the wholeocean.Every possible transitional form occurs between the barrier reef skirting themainland and the perfectly circular atoll lashed on its outer rim bj^ the stormyseas, and enclosing an inner lagoon of smooth water. Most of the forty thousandrocks and islets in the Maldive Archipelago are so disposed as to form atollswithin 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 boldgenerali.-ations on the slow oscillations of the terrestrial crixst. Finding that thebarrier reefs and outer walls of the atolls rise in many places above deep waters,he concludes that these rocks were entirely built by thesame polypi who are stillpiling up similar structures. But as they can work only in the surface waterswhere the ceaseless ebb and flow brings them the materials of their edifices,' thegreat elevation of so many coralline rocks would seem to attest a gradual subsidenceof the marine level. The first colonies began their operations within about120 feet of the surface ;but according as the structures rose the ground sank, and