CORAL REEFS OF THE INDIAN OCEAN. 



17 



islands, are both of coral formation with no evidence of change of level. Platte island to 

 the south lies on a bank, 17 miles long, separated by a channel, 25 miles wide ; it has 

 a low coral island, apparently on a reef awash, having a lagoon. Coetivy, 60 miles S.E., 

 is a low coral island with sand hills, .5| miles long, by 1^ broad, on a bank of about 

 twice the size ; near it and in the dividing channel towards the Seychelles are several other 

 banks not properly surveyed. Continuing the main line to the S.W., separated fi-om the 

 Seychelles by a 22 mile channel with 11.50 f. of water, is the Amirante bank, running 

 89 miles N.N.E. and S.S.W., varying in breadth from .5 miles at the north end to 23 miles 

 at the south. There are nine principal reefs and islands, which tend to lie near the edge 

 of the bank ; none of them attain a height of even 20 feet. St Joseph and Desroches are 

 atolls with soundings of 2 and 17 /. respectively in their lagoons, the rest being mostly 

 round reefs, drying all over at low tide, with mere sand cay islands. Separated from the 

 last by a channel, 46 miles across, with one sounding of 9.52 f. nearly in the middle, are 

 two reefs, Alphonse and Francois; the former is an atoll with basin 10/' deep, and the latter 

 a reef, 8^ miles long by 5 broad, with two islands, drying in patches, a little hollowed in the 

 centre, but not enough to form a definite lagoon. 



3. Chagos Section. 



The Chagos Archipelago differs from either of the preceding sections, in that all its 

 islands and reefs are of coral forma- 



SpoallersBaiik /*^ J\ 



tion alone, hence bearing a much 

 closer comparison to the Maldives. 

 It lies almost in the same latitude 

 as the south of the Seychelle group, 

 but 600 miles further eastwards 

 than that line of reefs ; no sound- 

 ings exist between. The group 

 covers an area of about 170 miles 

 north and south by 110 miles east 

 and west, and is formed by five 

 atolls, three atoll banks and three 

 other banks of no very determinate 

 shape ; there are no bottom sound- 

 ings between the different atolls 

 and banks, a line of more than 

 150 /. being seldom used in Mores- 

 by's time (1835), the usual being 

 about half this. 



To the S.S.W. lie three banks : — 

 Centurion, 5 miles long by 2 broad, 

 rim 7 to 10/. deep, a single .sound- 

 ing of 14 y! in the central basin ; 

 Ganges, 4 miles long by 3 broad, 

 rim 8 to 12/ deep, a single sound- 

 ing of 12/ in the centre; and Pitt, 

 G. 









-•S loo BO 



lOO 



•35 



1-^ 



200 





140 







" ~ ii 19 ' 

 " w S, ; 



*='3€.„r «»..?,,.* J v~;7«/ 



-:..-.■■•' ,^. 



'^^ Great 

 Chagos Bank I86e 



•?■•* ^, 



*,'■».-» 



ilea 



jf^a la 7 . 



ISO 



^v>_ 



^"^r 



bO 



lit J " «7 e^ 

 KttBank •,» S. 'e. 



J*6 



IGO 



Ga/tffesE^a. 



136 



I'M 

 Bank 6 



(40 i£o 



°^ 233 



' 200 



330 



ISO 



J80 



Fig. 4. Chagos Archipelago. {From the Admiralty Chart.] 



