8 DESCRIPTION OF THE PLATES. 



not be introduced ; the nautical mark (— ) over the figures 

 250 and 200, between Mahlos Mahdoo and Horsburgh atoll 

 and Powell's island, signifies that soundings were not 

 obtained at these depths. 



Fig. 5. — Bow, or Heyou ATOLL (or lagoon-island), in the Low 

 Archipelago, from the survey by Capt. Beechey, R.N. ; 

 the lagoon is choked up with reefs, but the average greatest 

 depth of about 20 fathoms, is given from the published 

 account of the voyage. 



PLATE IV. 



Fig. 1. — Great Chagos Bank, in the Indian Ocean; taken 

 from the survey by Capt. Moresby and Lieut. Powell ; 

 the parts which are shaded, with the exception of two 

 or three islets on the western and northern sides, do 

 not rise to the surface, but are submerged from 4 to 

 10 fathoms ; the banks bounded by the dotted lines lie 

 from 15 to 20 fathoms beneath the surface, and are 

 formed of sand ; the central space is of mud, and from 

 30 to 50 fathoms deep. 



Fig. 2. — A vertical section, on the same scale, in an E. and W. 

 line across the Great Chagos Bank, given for the sake of 

 exhibiting more clearly its structure. 



Fig. 3. — Peros Banhos atoll (or lagoon-island), in the 

 Chagos group in the Indian Ocean ; from the survey by 

 Capt. Moresby and Lieut. Powell ; not nearly all the 

 small submerged reefs in the lagoon are represented; the 

 annular reef on the southern side is submerged. 



PLATE V. 



The principles on which this map was coloured are explained 

 in the beginning of Chapter VI. ; and the authorities for 

 each particular spot are detailed in the Appendix. The 

 names printed in italics in the Index refer to the Appendix. 



