134 CORAL FORMATIONS. 
Maldives have deeper lagoons than the northern, fifty or sixty fathoms 
being found in them. This fact indicates that subsidence was pro- 
bably most extensive to the south, and perhaps also most rapid. The 
sinking of the Chagos Bank still further south, in nearly the same 
line, may therefore have some connexion with the subsidence of the 
Maldives. 
In view of the facts which have been presented, it appears that the 
coral atoll once formed a fringing reef around a high island. The 
fringing reef, as the island subsided, became a barrier reef, which 
- continued its growth while the land was slowly disappearing. ‘The 
area of waters within finally contained the last sinking peak: another 
period, and this had gone—the island had sunk, leaving only the 
barrier at the surface and an islet or two of coral in the enclosed 
lagoon. Thus the coral wreath thrown around the lofty island to 
beautify and protect, becomes afterwards its monument, and the only 
record of its past existence. The Paumotu Archipelago is a vast 
island cemetery, where each atoll marks the site of a buried island. 
The whole Pacific is scattered over with these simple memorials, and 
they are the brightest spots in that desert of waters. 
5. GEOGRAPHICAL DISTRIBUTION OF CORAL REEFS AND ISLANDS. 
The distribution of coral reefs over the globe depends on the follow- 
ing circumstances, arising from the habitudes of polyps already ex- 
plained : 
1. The temperature of the ocean. 
2. The character of coasts as regards (a) the depth of water,—(d) 
the nature of the shores,—(c) the presence of streams. 
3. Liability to exposure to destructive agents, such as volcanic heat. 
It has been stated that reef-growing corals will flourish in the hottest 
seas of the equator, and over the ocean wherever the winter tempe- 
rature is not below 66° F. The isocheimal line of this temperature 
therefore forms the boundary line of the coral-reef seas. 
This line traverses the oceans between the parallels 26° and 30°, or 
in general near 28°. But in the vicinity of the continents it under- 
goes remarkable flexures, from the influence of oceanic currents, the 
polar currents bending it towards the equator, while the tropical cause 
