CHAPTER I. 
ATOLLS OE LAGOON-ISLANDS. 
SECTION FIRST, KEELING ATOLL. 
Corals on the outer margin — Zone of Nulliporce — Exterior reef — 
Islets —Coral-conglomerate — Lagoon — Calcareous sediment — 
Scari and Holuthurice subsisting on corals — Changes in the con- 
dition of the reefs and islets — Probable subsidence of the atoll — 
Future state of the lagoon. 
Keeling or Cocos atoll is situated in tlie Indian Ocean, 
in 12° 5 ' S. and long. 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 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 
