Volume 14 Australasia - dana ward's homepage

CORAL FORMATIONS. 83so the reef continually subsiding at the base and rising at the summit, grew to afar greater thickness than 120 feet.Thus was explained the formation of barrier reefs at great distances from theshore. At one time they fringed the coast, which slowly sank with the generalmovement of subsidence, while the reefs continued awash, thanks to the incessantlabour of their coralline inhabitants. The mainland, which formerly served tosupjDort the superstructure, gradually sank deeper and deeper, thus continuallyretiring from the outer barrier of the steadily rising coral reefs. The passagealso became gradually enlarged, and by the disappearance of the central nucleusitself the inner waters were at last transformed to a lagoon. Certain archipelagoes,such as the Low Islands, are compared by Dana to a vast cemetery,where every atoll marks the site of an engulfed land.According to this theory it would therefore be easy to determine the characterof the oscillating movements to which the oceanic islands are subjected. Thereefs raised to great heights above the sea would thus indicate an area of upheaval,the fringing coralline rocks would imply a state of comparative stability on theseaboards, while the barriers and the atolls might be likened to floats placed on thesites of submerged lands.Most of the Pacific islands—that Is to say, all those thatfoUow from Pitcairn In the Low Archipelago to the Philippines along a Hnepassing north of Tahiti and Samoa—would thus belong to a zone of depression, andthese scattered groups might be regarded as fragments of a vanished continent,stretching across the south side of the North Pacific Ocean.Such is Darwin's theory, which, however, can scarcely be applied with anyprobability to all the oceanic lands girdled by coral reefs. "Wherever the rockypedestals supporting the superstructures of living polypi themselves consist ofcalcareous secretions to any great depth, there can be no doubt that subsidence hasreally taken place. But verifications have hitherto been made only at a limitednumber of points, and in the absence ofdirect observations It would be rash to domore than regard subsidence as very probable wherever the outer walls of thecoralline Islands plunge rapidly— as, however, they rarely do—Into abysmal depths.Thus near Enderbury, In the Phoenix Archipelago, the soundings reveal 1,800fathoms within 3 miles of the shore, 900 fathoms at 1,400 yards from DangerIsland, near Vanikoro, while one of the reefs at Tahiti Indicates a seaward slope of72 degrees.On the other hand, observations made in the vicinity of certain coralline islandsshow that at the foot ofan escarpment less than 200 feet high, there stretch vastplatforms where fragments of volcanic origin have been found scattered amongstcrumbling blocks of coral. In this case it is qiilte possible that eruptive coneseroded by the waves to a slight depth below the marine surface may have servedas foundations for the coral-builders, or else that their structures have been raisedon rocks entirely formed by other organisms working at considerable depths. Butmany protracted observations must still be made before the diverse coralline Islandscan be classified according to their origin and history. Several groups, such asthe Low Archipelago, Fiji, the Pelew, Solomon, and Tonga Islands, supposed byVOL. XIV, I)