OCTOBEK 6, 1905] 



NA rURE 



571 



THE PERCY SLADEty EXPEDITION IN H.M.S. 

 SEALARK. THE CHAGOS ARCHIPELAGO. 



t~\ UR arrival in Mauritius on August 5 completed the 

 ^^^ first half of our cruise in H.M.S. Scalark, together 

 with all our work directly connected with the Chagos 

 Archipelago. This work may be divided under two heads, 

 oceanography and biology. The former has been carried 

 out mainly by Commander Boyle Somerville and his officers 

 in view of the scientific objects of the expedition, but at 

 the same time it is all of practical value for navigation 

 in these waters. In many respects it has been of a 

 singularly arduous nature; surveys by camping parties and 

 deep soundings from the ship have been cajried on simul- 

 taneously, together with numerous observations on the 

 tides, currents, sea temperatures, &c. To a considerable 

 gree it and all the work has been hampered by the 



sand here and th( re. Indeed, our evidence points to the 

 impossibility of iniy upwtirti growth being in progress 

 between the different Chagos banks, and to the probability 

 of considerable current being felt even at 500 fathoms. 



The reefs of the Chagos are in no way peculiar save 

 in their extraordinary paucity of animal life, to which I 

 referred in m.y last letter. Green weed, too, of every 

 sort is practically absent. However, this barrenness is 

 amply compensated for by the enormous quantity of nulli- 

 pores (Lithothamnia, &c.), incrusting, massive, mammillated, 

 columnar, and branching. The outgrowing seaward edges 

 of the reefs are practically formed by their growths, and 

 it is not too much to say that were it not for the 

 abundance and large masses of these organisms there 

 would be no atolls with surface reefs, &c., in the Chagos. 

 The lagoon shoals of Egmont are covered by them', and 

 alone reach the surface ; having once done so they di 



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■a* so 



'fec**^; 





Eagltlff 



Chagos Bank ISOS 



heavy weather, which, contrary to all expectation, we have I and become hollowed out in the centre, finally resembling 



experienced, winds from south to east with heavy, con- i miniature atolls. 



fused seas, partially induced by the com- I 



paratively shallow waters of the Chagos 



Archipelago, and partially due to the current, 



which set in an easterly direction (against the 



wind) during the whole time we were in the 



group. 



It is almost too soon to attempt to sum- 

 marise any of the results of the cruise, but 

 the soundings taken on our course from 

 Ceylon to the Chagos and from the latter to 

 .Mauritius show that the archipelago is closely 

 surrounded, both to the north and west, by 

 the 2000-fathom line, and that there is at the 

 present day no trace in the topography of 

 the Indian Ocean of any former connection of 

 the group with either the Maldives or the 

 banks on the Seychelles-Mauritius line. The 

 Chagos Archipelago appears, indeed, to stand 

 by itself, being built up on a plateau rising to 

 a depth of 800 fathoms in an ocean of an 

 average depth of 2300 fathoms. Previously 

 there were no bottom soundings between the ■■ 



banks and shoals of the group, but now a 

 large series (more than 100) have been run, ^ 



showing depths of 400 fathoms to 800 fathoms 

 between the individual banks ; from most of 

 these a sample of the bottom has been 

 obtained. 



Broadly speaking, the Chagos group may be K« 



said to consist of three atolls to the north 

 (Salomon, Peros Banhos, and Blenheim), the lio 

 Great Chagos Bank in the centre (60 miles by 

 90 miles), and to the south two atolls, Diego '5t> ^ 

 Garcia and Egmont, besides certain submerged ; 



banks both to the north and south. Of these, ^ 



H.M.S. Sealark has re-charted Salomon and caaurio 

 parts of Peros Banhos, while Cooper and I ,j>,'°'"*,&" 

 have in addition examined the southern atolls. 

 Salomon was very carefully surveyed, our iSo 



intention being to make a comparison between 

 its condition at the present time and when 

 Powell's chart was made in 1837. The 

 latter chart, however, proved to have been so carelessly 

 drawn that any close comparison is, I fear, useless, 

 but the new chart should be of great value when it is 

 possible to re-examine the atoll at some future date. 

 Its section lines show that it arises in the last 400 fathoms 

 by similar slopes to those of Funafuti, but it is a much 

 simpler atoll, having only one passage, and more than half 

 its reef crowned by land. Our numerous soundings and 

 dredgings on its slopes leave no room for doubt but that 

 its present reef is extending outwards on every side on 

 its own talus, in fact, that the steep found round it (and, 

 indeed, most atolls) is, in this instance, simply the slope 

 at which coral and other remains from the reef above come 

 to rest in the water. Its face was everywhere singularly 

 barren; Lithothamnion, Polytrema, and, of course, reef- 

 corals were not obtained below 50 fathoms. Further out, 

 at 250 fathoms and over, the bottom was smooth and 

 barren ; the lead constantly failed to bring up any samples, 

 ■while the somewhat broken and dented, but almost empty, 

 dredges gave the idea of bare rock with a little muddy 

 NO. 1875, VOL. 72] 



-Chait of the Chagos .^rc 



In 



reefs 

 ablv. 



uch a large group the conditions of the encircling 

 against the lagoons naturally vary very consider- 

 In general their inner edges reach the surface, and 

 in the more open atolls the lagoon slope to 10 fathoms 

 closely resembles the seaward slope. The bottoms of the 

 lagoons are bare, rock, hard sand or mud, with shoals 

 arising precipitously here and there, built up by a few 

 species of coral, but largely covered by Xenia and Sarco- 

 phytum (as also are the only two submerged banks, Wight 

 and Centurion, which we examined). Diego Garcia lagoon 

 differs somewhat owing to its being almost completelv 

 surrounded by land. It has perhaps the most varied fauna 

 in the group, and alone gives definite evidence of enlarging 

 in every direction. Everywhere the land is entirely of coral 

 origin. Diego Garcia shows signs of a recent elevation 

 of a few feet, the present single island having been formed 

 by the joining up of a series of separate islets on an 

 elongated reef. The kuli or barachois (large shallow lakes) 

 of the same island owe their origin to the same elevation, 

 though elsewhere in the group they are generally due to 



