56 



NA TURE 



[July S, 1922 



Coral Reefs of the Louisiade Archipelago. 1 



By Prof. W. M. Davis, Harvard University. 



T 1 



'HE Louisiade archipelago, consisting of four 

 medium-sized and many small islands east of 

 New Guinea, is well represented on British Admiralty 

 chart 2124 on a scale "of about i : 280,000 ; chart 1477 

 shows part of the archipelago in greater detail on 

 a scale of about 1 : 140,000. According to brief 

 accounts by Macgillivray,'- Thomson, 3 and Maitland, 4 

 the chief islands are composed of steeply inclined and 

 deeply eroded schists and slates, traversed by quartz 

 veins ; they are evidently parts of the mountain 

 range that extends for hundreds of miles along the 

 northern coast of New Guinea, from which they have 

 been separated by strong subsidence after having been 

 eroded to about their present form. The largest 

 island is Tagula, 30 miles in length east-west along 

 the trend of its schists, and 8 or 9 miles in width ; it 

 has an embayed shore line and rises in ten summits to 

 heights of from 1330 to 2645 feet. Near by is the 

 Calvados chain of satellite islands, which begins about 

 7 miles north of the middle of TaguJa and extends 70 

 miles westward ; it includes more than a score of 

 members, the largest having a length of n miles and 

 a height of n 10 feet. Tagula and its chain of satel- 

 lites are enclosed by a superb barrier reef, the irreg- 

 ularly oval circuit of which measures 112 miles in 

 east-west diameter by about 30 miles north-south ; it 

 is unquestionably one of the finest reefs of its kind in 

 the whole Pacific. 



The smaller islands of Rossel to the east and De- 

 boyne to the north-west of Tagula are also surrounded 

 by sea-level reefs, partly as fringes but mostly barriers. 

 Misima, north of Deboyne, measuring 22 by 10 miles 

 and reaching 3500 feet in height, is peculiar in having 

 no sea-level reefs and in descending rapidly into deep 

 water, although it is terraced by unconformable reefs 

 at various altitudes. It has therefore suffered a 

 recent uplift after having previously taken part in the 

 subsidence which characterises the other islands ; but 

 its subsidence must have been more rapid than theirs 

 as it has no widely developed barrier-reef lagoon floor, 

 either near present sea-level or above or below it. 



The Tagula barrier reef and its great lagoon merit 

 special attention from the evidence that they give 

 regarding the verity of certain coral-reef theories. 

 The reef is best developed around the south-eastern 

 or windward half of its great oval circuit, where it is 

 interrupted by only four passes in a curved distance of 

 no miles, and where the reef flat has a width of 2 or 

 3 miles. The north-western or leeward half of the 

 barrier is strikingly discontinuous and consists in part 

 of small patches, but more commonly of atoll-like 

 loops and rings, thirty-six in number, from 1 to 5 miles 

 in diameter, enclosing little lagoons from 10 to 17 

 fathoms in depth. The loops and rings of this half 

 of the circuit are separated by as manv passages, from 

 I to 3 miles wide and from 15 to 35 fathoms deep. 

 But the most remarkable features of this part of the 

 barrier are the small or minute but high islands, here 

 to be referred to as outposts, which rise in twenty-two 

 of the reef loops. The largest of them is only 4 miles 

 in diameter ; their heights vary from 40 to 530 feet. 

 Some of them appear to consist of schist, judging by 



1 Reprinted from the Proceedings of the National Academy of St iem es' 

 Washington, D.C., U.S.A. (vol. 8, No. i. fan 



- I. Macgillivray, "Narrative of the Voyage of H.M.S. Rattlesnake," 

 London. 1S52, 2 vols. See i. 1S2 ; ii. 72. 



■ B. H. Thomson, " New Guinea : Narrative of an Exploring Expedition 

 to the Louisiade and D'Entrecasteaux Islands," Proc. Roy. Geogr. Soc, 

 n, 1889 (525-542). 



4 A. G. Maitland, " Geological Observations in British New Guinea," 

 Queensland, Geol. Surv. Pub., 85, 1892. " Salient Geological i 

 New Guinea," Journ. \V. Austral. X. H. Soc, 2, 1905 (32-s.o). 



NO. 2749, VOL. I IO] 



their trends ; but according to Maitland some of the 

 others are volcanic and a few are made of limestone. 

 As elements of a barrier reef, these small but high out- 

 post islands are so exceptional as to be almost unique. 



The great Tagula lagoon is divided by the Calvados 

 chain of satellite islands into a smaller northern and 

 a larger southern compartment ; the northern com- 

 partment is of triangular outline, with its base along 

 the dividing chain and its vertex about 10 miles awav 

 at the most northern point of the reef ; it occupies 

 about one-sixth of the entire reef-enclosed space, which 

 is about 2000 square miles in total area. The southern 

 compartment measures 20 miles across, and extends 

 east-west along the whole 112 miles of the lagoon 

 length ; it occupies about four-sixths of the enclosed 

 area ; the remaining sixth is taken by Tagula and the 

 satellite islands. The greater part of the lagoon floor 

 in both compartments is a gently undulating plain 

 usually from 25 to 35 fathoms in depth. The depth 

 of the southern compartment increases gradually for 

 a moderate distance from the broad enclosing reef, 

 and more rapidly from the islands of the Calvados 

 chain. The greatest depths, 46 fathoms in the 

 southern or windward compartment and 49 fathoms 

 in the northern or leeward compartment, are in both 

 cases found much nearer the dividing island chain 

 than the outer barrier reef. The exterior slopes of the 

 reef fall off rapidly into deep water ; a few soundings 

 show depths of more than 600 fathoms two miles 

 from the reef on the west and north-west. 



A correct theory of the Louisiade reefs must take 

 account of the great subsidence that the islands have 

 suffered. It would therefore appear that the present 

 sea-level reefs should be regarded as the successors of 

 a long-lived series of upgrowing reefs which have been 

 formed, essentially according to Darwin's theory, by 

 more or less intermittent upgrowth from earlier shore 

 lines of the subsiding mountainous islands. It is 

 probable that where the island slopes were very steep, 

 the reefs, presumably inclining inwards as they grew 

 up, remained attached to the shore as fringes ; con- 

 versely, where the island slopes were gentler or where 

 low slopes have been broadly submerged, the reefs 

 now form offshore barriers. During the upgrowth of 

 the reefs, some of their detritus must have been swept 

 seaward, to form the submarine talus that descends 

 into deep water ; the rest must have been swept into 

 the lagoons, where, reinforced by local organic detritus 

 and probably in smaller measure by detritus from the 

 islands, it appears to have aggraded the " moats " 

 between the reefs and the islands. 



It thus seems that the formation of the great under- 

 mass of the Louisiade reefs, and especially of the 

 Tagula reef, may well have been consistent with the 

 conditions and processes of Darwin's theory. It 

 should be added that the evidence for the strong 

 subsidence of the Louisiade islands is, in view of their 

 constitution, much more direct than that furnished 

 for the similar subsidence of most reef-encircled 

 volcanic islands in the central Pacific ; and that this 

 well-certified subsidence of the foundations on which 

 the Louisiade reef-masses have been built up gives 

 immensely greater support for Darwin's theory than 

 is afforded by the atolls of the open Pacific, where the 

 occurrence of subsidence is indicated only by indirect 

 evidence. It remains to inquire whether the Louisi- 

 ade sea-level reefs, which surmount the great under- 

 mass, accord with or contradict other coral-reef 

 theories, especially the newly framed Glacial-control 

 theory of sea-level reefs. This theory was proposed 



