CHAPTEK I. 



ATOLLS OR LAGOON-ISLANDS. 



SECTION FIRST, KEELING ATOLL. 



Corals on the outer margin — Zone of Nulliporcs — Exterior reef — Islets — 

 Coral-conglomerate — Lagoon — Calcareous sediment — Scari and Holu- 

 thurios 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 long. 90° 55' E. : a reduced chart of it 

 from the survey of Capt. FitzKoy and the officers of 

 H.M.S. Beagle, is given in Plate I. fig. 10. 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 wood^ 

 cut (No. 3) 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. The section is true to the scale 

 in a horizontal line, but it could not be made so in a 

 vertical one, as the average greatest height of the land 

 is only between six and twelve feet above high-water 

 mark. I will describe the section, commencing with 



