Sect. II. 
RATE OF GROWTH. 
99 
and tlieir barrier-reefs are now in parts 300 feet above 
the level of -the sea. 1 
Some attempts have been made, with but little 
success, to ascertain by boring the thickness of coral 
formations. At Bow Island, in the Low Archipelago, 
Sir E. Belcher 2 bored to a depth of 45 feet, and 
below the first 20 found only coral-sand. During 
Wilke’s Expedition, 3 in a boring of 21 feet in depth 
on one of the islands in the same archipelago, coral- 
sand was passed through for the first 10 or 11 feet, 
and then solid reef rock. On one of the Maldiva 
atolls in the Indian Ocean, Captain Moresby bored ter 
a depth of 26 feet, when his auger broke. lie gave 
me the matter brought up, and it was perfectly white 
like finely triturated coral-rock. 
In my description of Keeling atoll, I have ad- 
vanced some facts showing that the reef has probably 
grown outwards; and I found, just within the outer 
margin, the great mounds of Porites and of Millepora, 
with their summits lately killed, and their sides sub- 
sequently thickened by the growth of the coral : a 
layer, also, of Nullipora had already coated the dead 
surface. As the external slope of the reef is the same 
round the whole of this and many other atolls, the 
angle of inclination must result from an adaptation 
1 Dana, Corals and Coral Islands, p. 336. Also Forster’s Voyage 
round the World with Cook, vol. ii. pp. 163, 167. Williams’s Narra- 
tive of Missionary Enterprise, pp. 30, 48, and 249. 
2 Voyage Round the World, vol. i. 1843, p. 369. 
3 Narrative U.S. Exploring Expedition, vol. iv. p. 268. Dana, 
Corals and Coral Islands, p. 184. 
