DESCRIPTION OF PLATES. XIX 



Powell ; scale \ of an inch to a mile ; not nearly all the small sub- 

 merged reefs in the lagoon are represented ; the annular reef on the 

 southern side is submerged. 



Fig. 10. — Keeling, or Cocos atoll (or lagoon-island), in the Indian 

 Ocean ; from the survey by Capt. FitzKoy ; scale £ of an inch to a 

 mile ; the lagoon south of the dotted line is very shallow, and is left 

 almost bare at low water ; the part north of the line is choked up 

 with irregular reefs. The annular reef on the N.~W. side is broken, 

 and blends into a shoal sand-bank, on which the sea breaks. 



PLATE II. at end of Volume. 



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

 survey by Capt. Moresby and Lieut. Powell ; scale ^ of an inch to 

 a mile (same scale as Hogoleu, in Plate I.) ; 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. — Menchicoff atoll (or lagoon-island), in the Marshall Archi- 

 pelago, northern Pacific Ocean ; from Krusenstern's atlas of the 

 Pacific ; originally surveyed by Capt. Hagemeister ; scale ± of an 

 inch to a mile ; the depth within the lagoons is unknown. 



Fig. 4.— Mahlos Mahdoo atoll, together with Horsburgh atoll, in the 

 Maldiva Archipelago ; from the survey by Capt. Moresby and Lieut. 

 Powell ; scale ± of an inch to a mile ; the white spaces in the 

 middle of the separate small reefs, both on the margin and in the 

 middle part, are meant to represent little lagoons ; but it was found 

 not possible to distinguish them clearly from the small islets, which 

 have been formed on these same small reefs ; many of the smaller 

 reefs could 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. 



