183G.] • OR ATOLLS. 467 



plicable to the northern Maldiva atolls in the Indian Ocean 

 (one of which is 88 miles in length, and between 10 and 20 in 

 breadth), for they are not bounded like ordinary atolls by narrow 

 reefs, but by a vast number of separate little atolls ; other little 

 atolls rising out of the great central lagoon-like spaces, A third 

 and better theory was advanced by Chamisso, who thought that 

 from the corals growing more vigorously where exposed to the 

 open sea. as undoubtedly is the case, the outer edges would grow 

 up from the general foundation before any other part, and that 

 this would 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 important consideration has been overlooked, 

 namely, on what have the reef building corals, which cannot live 

 at a great depth, based their massive structures ? 



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, invariably came up marked with the impressions of living 

 corals, but as perfectly clean as if it had been dropped on a 

 carpet of turf ; as the depth increased, the impressions became 

 less numerous, but the adhering particles of sand more and more 

 numerous, until at last it was evident that the bottom consisted 

 of a smooth sandy layer : to carry on the analogy of the turf, 

 the blades of grass grew thinner and thinner, till at last the soil 

 was so sterile, that nothing sprang from it. From these obser- 

 vations, confirmed by many others, it may be safely inferred 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 fragments, and the winds pile up sand. 

 Thus the Kadack group of atolls is an irregular square, 520 

 miles long and 240 broad ; the Low archipelago is elliptic-formed, 

 S40 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 mileij in lenofth, in which not one sinole island rises above 

 the specified height. Again, in the Indian Ocean there is a 

 space of ocean 1500 miles in lergth, including three archi- 



