CHAPTER I. 



ATOLLS OR LAGOON-ISLANDS. 



Section First. — Keeling Atoll. 



Corah on the outer margin. — Zone of Nulliporce. — Exteridf reef — Islets. 

 — Coral-conglomerate. — Lagoon. — Calcareous sediment, — Scari and 

 Holuthurice subsisting on corals. — Changes in the condition of the 

 reefs and islets. — Probable subsidence of the atoll. — Future state of 

 the lagoon. 



Keeling or Cocos atoll is situated in the Indian Ocean, 

 in 12° 5' S., and longitude 90 55' E. : a reduced chart of 

 it from the survey of Capt. Fitzroy and the Officers of 

 H.M.S. Beagle^ is given in Plate II., Fig. 2. The greatest 

 width of this atoll is nine miles and a half. Its structure 

 is in most respects characteristic of the class to which it 

 belongs, with the exception of the shallowness of the lagoon. 

 The accompanying woodcut represents a vertical section, 

 supposed to be drawn at low water from the outer coast 

 across one of the low islets (one being taken of average 

 dimensions) to within the lagoon. 



A.— Level of the sea at low water : where the letter A is placed, the depth is 

 25 fathoms, and the distance rather more than 150 yards from the edge of the 

 reef. 



