INDIAN OCEAN. 241 



Chagos, Maldiva, and Laccadive Archipelagoes. — These 

 three great groups of atolls and atoll-formed banks, have been 

 often referred to in this volume, and are now well known from 

 the admirable surveys of Captain Moresby and Lieut. Powell. 

 Their published charts are worthy of the most attentive ex- 

 amination. In the Laccadive group, the atoll-like structure 

 is less evident than in the Maldivas ; nevertheless the islands 

 are all low, not exceeding the usual height of coral formations 

 (see Lieut. Wood's account, Geograph. Journ. vol. vi. p. 29), 

 and most of the reefs are circular ; within several of them, as 

 I am informed by Captain Moresby, there is deepish water ; 

 these, therefore, have been coloured blue. Directly north of 

 the Laccadives, and almost forming part of the same group, 

 there is a long, narrow, slightly-curved bank, rising out of 

 the depths of the ocean, composed of sand shells and decayed 

 coral, with from 23 to 30 fathoms on it. I have no doubt 

 that it has had the same origin with the other atoll-like 

 banks; but as it does not deepen towards the centre, I have 

 not coloured it. I might have referred to other authorities 

 regarding these three archipelagoes ; but after the publication 

 of the charts by Captain Moresby (to whose personal kind- 

 ness in giving me much information I am exceedingly in- 

 debted), this would have been superfluous. 



The Sahia de Malha Bank consists of a series of narrow 

 banks, with from 8 to 16 fathoms on them; they are arranged 

 in a semi-circular manner, round a space about 40 fathoms 

 in depth, sloping to the S.E. to unfathomable depths; they 

 are steep on both sides, but more especially on the ocean- 

 side. Hence this bank closely resembles in structure, and 

 I may add from Captain Moresby's information in composition, 

 Pitt's Bank in the Chagos group ; and Pitt's Bank must, from 

 what we know about the great Chagos Bank, be considered 

 as a sunken, half- destroyed atoll; hence coloured blue. — 

 Cargados Carajos Bank : its southern portion consists of a 

 large, curved coral-shoal, with some low islets on the eastern 



R 



