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NA TURE 



[January 25, 1906 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nati re. 

 No notice is taken of anonymous communications.] 



The Percy Sladen Expedition in H.M.S. " Sea'aik " 

 to the Indian Ocean. The Seychelles Archipelago > 



After leaving the Sealark and dispatching our collec- 

 tions home, Mr. Forster Cooper and I spent seven weeks 

 in exploring the Seychelles Archipelago as thoroughl) as 

 possible, dividing our time between Praslin and Main- as 

 centres. We camped for eighteen days on the former 

 island, and then separated, Mr. Cooper being responsible 

 for Silhouette Island, the fauna of which appeared to be 

 almost unknown, while I visited various parts of Mahe 

 and examined its reefs and neighbouring islets. Un- 

 fortunately the weather had been exceedingly dry, and 

 continued to be so during the first half of our stay, while 

 it was correspondingly wet during the second half. As 

 a result, land collecting was at all times extremely 

 laborious, and insects were throughout found to be scarce, 



-Seychelles Bank showing the 



.-falhom lini 



both in species and number. Other groups of land animals 

 we believe to have been fairly thoroughly collected. 



The Seychelles Archipelago comprises a number of islands 

 arising on a submarine bank, which extends in a more or 

 less east and west direction, about 190 miles long by 

 too miles broad (Fig. 1). It was fairly regular in contour 

 save to the south-east, where a horn stretches out for some 

 distance along the line towards Mauritius. Separated 

 from this same projection by deeper water are three 

 similar but smaller banks, of which that of Coetivy alone 

 has land, and in correspondence there is to the south-west 

 the Amirante Bank with its little group of islands. 



The Seychelles Bank itself has an average depth of 

 30 fathoms, and our soundings off it to the north-west, 

 east, and south show that it has a contour similar to 

 those of typical coral reefs and banks. An outer rim is 

 indicated along the whole of its north-western half by a 

 series of shallower soundings, but to the south-east the 

 depth does not markedly shoal (Fig. 1). It has two typical 

 surface-reefs with coral islets to the north, Bird and 

 Dennis, but elsewhere the rim is generally covered by at 

 least 7 fathoms of water. Between these two islands, and 

 to the west of Bird, the character of the bottom is some- 



1 For earlier repons see Nature, April 13, August 10, October 5, 

 November o, and December 21, 1005. 



NO. 1891, VOL. 73] 



what similar to what we found at the edges of the sub- 

 merged Saya de Malha and Nazareth Banks, and indicates 

 the upgrowth of a rim. The fauna was more varied, but 

 the bottom was less covered by nullipores and corals, and 

 a few green algse were abundant. All sedentary 

 organisms were covered with dirt and unhealthy between 

 Bird and Dennis Islands, where there would appear to be 

 a natural outfall for the tidal and other currents. In this 

 position there is certainly no upgrowth of the rim, while 

 elsewhere it must be exceedingly slow. The bottom within 

 the rim is sand, muddy sand, or mud, the latter held 

 together by the roots of algse. Strong currents sweep 

 across it, and even during our visit, between the two 

 trades in dead calm weather, the sea-water was always 

 cloudy, so that, except in favoured spots, corals could 

 scarcely grow up into reefs. 



The islands of the Seychelles naturally divide themselves 

 into two groups to the west and east, with Mahe and 

 Praslin as centres (Fig. 2). The former comprises Mahe, 

 Silhouette, and North, with a series of small islets around 

 the first, outlying buttresses and peaks of the same, with 

 only a few fathoms of water between. Praslin also is 

 similarly surrounded by a series of tiny islets and rocks, 

 but in addition there are eleven other islands in its group 

 (of which we visited five), separated 

 by considerable channels. Mahe. and 

 Silhouette attain heights respectively 

 of 2993 feet and 2407 feet, but Praslin 

 and the eastern islands do not exceed 

 12711 feet. All islands were found to 

 be formed of similar, coarse granites 

 (or granulitic quartzes), with narrow, 

 vertically extending dykes of finer 

 grained black rock, apparently a 

 variety of granite, along which the 

 mountain streams have invariably cut 

 their courses. In addition, many of 

 the islands have against their coasts, 

 in bays and suitable situations, flats 

 of sand, largely coraliferous. Some 

 of these have doubtless been formed 

 by a washing up of sand from the 

 sea, and some are partially at least 

 of delta formation, but in places there 

 is evidence of a recent elevation of 

 more than 30 feet. On the island of 

 Silhouette, Mr. Cooper in five situ- 

 ations found masses of coral rock, 

 cemented on to the granite, at various 

 heights between 15 feet and 30 feet 

 above the low-tide level, and around 

 the coasts of Mahe and its smaller 

 islets there is evidence of a similar 

 upheaval. Besides this, there are in- 

 dications (particularly in Mahe) of an 

 ancient elevation of upwards of 200 

 feet. Definite rocks belonging to it 

 do not, so far as I could find, still exist. Marine action 

 prior to its occurrence perhaps accounts for the almost 

 continuous line of precipices, which at various distances 

 from the present coast extends along the eastern or wind- 

 ward side lit Main-. The question, however, requires 

 further investigation. In any case, the deep canons of 

 many of the mountain brooks, and the very extensive 

 weathering of the granites, indicate the not inconsiderable 

 antiquity of the land as such. 



Barrier reefs nowhere exist around the islands of the 

 Seychelles Group, and fringing reefs only in certain some- 

 what protected situations (Fig. 3). Mahe Island extends 

 more or less north and south, with two conspicuous points 

 to the west. A fairly continuous reef lies along its east 

 side, but there is no reef off its north and south points, and 

 reefs only occur in the bays of its western side. Silhouette 

 is a round island with practically no reef, while Praslin 

 has reefs in its bays alone. La Digne, Curieuse, and the 

 smaller islands have merely patches of reef. On examin- 

 ing into the causes of this we found a luxuriance of coral 

 growth even off the points of the islands, but practically 

 a complete absence of nullipores. Indeed, these calcareous 

 algae are essential in the Indian Ocean for the consolidation 

 of corals into true reefs. Where fringing Hats actually do 

 occur, they would appear generally to consist of a basis 



