274 CORALS AND. CORAL ISLANDS. 
the erosion-made valleys of Tahiti would become deep bays, 
and above that of one thousand feet, fiord-like bays, with the 
ridges spreading in the water like spider’s legs; and this is a 
common feature of the islands and islets‘within the lagoons 
of barrier islands. The evidence of subsidence admits of no 
doubt. It makes the conclusion from the Gambier group 
positive ; and equally so that for Raiatea and Bolabola repre- 
and that 
for the Exploring Isles and others of the Feejee group; and 
> 
sented on the charts in Darwin’s “Coral Islands ;’ 
? 
that for islands, great and small, in the Louisiade Archipelago 
and in other similar groups over the oceans. 
Other arguments for the thickening of reefs through sub- 
sidence are afforded by the existence of elevated reefs, and of 
sunken and buried fringing reefs. 
The island of Metia is 250 feet in height (p. 195), full 
twice the coral-growing depth, and consists of horizontally 
stratified limestone. At the island of Mangaia, in the Her- 
vey group, the coral rock is raised 300 feet out of water. 
Such thick beds could not have been made by corals growing 
in depths not exceeding 150 feet without a sinking of scores 
of feet during their progress. 
Christmas Island, in the eastern part of the Indian Ocean, 
according to Mr. J. J. Lister and Captain Aldrich, R. N., al- 
though 1,200 feet high, has a series of horizontal terraces of 
coral-made rock to the top. There is at bottom a vertical 
cliff of 30 feet; then above the 120-foot level, a cliff of 85 
feet begins; above that of 475 feet, two cliffs together of 95 
feet; next a steep, rough slope for 650 feet of the height, 
ending in a top layer. Captain Aldrich, who ascended to the 
summit, says that he saw no rock but coral rock, and implies 
that the rock is in successive layers. The facts, taken as 
stated, prove a thickness of reef-rock of 1,200 feet. One 
