262 LAGOON-ISLANDS, OR ATOLLS. 



general foundation before any other part, and that 

 this wonhl account for the ring or cup-shaped 

 structure ; but we shall immediately see that in 

 this, as well as in the crater-theory, a most impor- 

 tant consideration has been overlooked, namely, on 

 what have the reef-building corals, which cannot 

 live at a great depth, based their massive struc- 

 tures 1 



Numerous soundings were carefully taken by 

 Captain Fitz Roy on the steep outside of Keeling 

 atoll, and it was found that within ten fathoms the 

 prepared tallow at the bottom of the lead invaria- 

 bly came up marked with the impressions of living 

 corals, but as perfectly clean as if it had been di-op- 

 ped on a carpet of turf; as the depth increased, 

 the impressions became less numerous, but the ad- 

 liering particles of sand more and more numerous, 

 until, at last, it was evident that the bottom con- 

 sisted of a smooth sandy layer : to carry on the 

 analogy of the turf, the blades of gi'ass gi-ew thin- 

 ner and thinner, till at last the soil was so sterile 

 that nothing sprang from it. From these observa- 

 tions, confirmed by many others, it may be safely 

 infen-ed that the utmost depth at which corals can 

 construct reefs is between 20 and 30 fathoms. 

 Now there are enormous areas in the Pacific and 

 Indian Oceans, in which every single island is of 

 coral formation, and is raised only to that height to 

 which the waves can throw up fi-agments and the 

 winds pile up sand. Thus the Radack group of 

 atolls is an irregular square, 520 miles long and 

 240 broad ; the Low Archipelago is elliptic-formed, 

 840 miles in its longer, and 420 in its shorter axis: 

 there are other small groups and single low islands 

 between these two archipelagoes, making a linear 

 space of ocean actually more than 4000 miles in 

 length, in which not one single island rises above 



