340 AREAS OF SUBSIDENCE. [CHAP. xx. 



others, this idea loses its plausible character; thus, Suadiva atoll is 

 forty-four geographical miles in diameter in one line, by thirty-four miles 

 in another line ; Rimsky is fifty-four by twenty miles across, and it has a 

 strangely sinuous margin ; Bow atoll is thirty miles long, and on an 

 average only six in width; Menchicoff atoll consists of three atolls 

 united or tied together. This theory, moreover, is totally inapplicable 

 to the northern Maldiva atolls in the Indian Ocean (one of which is 

 eighty-eight miles in length, and between ten and twenty 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 struc- 

 ture. 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 observa- 

 tions, confirmed by many others, it may be safely inferred that the 

 utmost depth at which corals can construct reefs is between twenty and 

 thirty 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 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 4,000 

 miles in length, in which not one single island rises above the specified 

 height. Again, in the Indian Ocean there is a space of ocean 1,500 

 miles in length, including three archipelagoes, in which every island 

 is low and of coral formation. From the fact of the reef-building corals 

 not living at great depths, it is absolutely certain that throughout these 

 vast areas, wherever there is now an atoll, a foundation must have 

 originally existed within a depth of from twenty to thirty fathoms from 

 the surface. It is improbable in the highest degree that broad, lofty, 

 isolated, steep-sided banks of sediment, arranged in groups and lines 

 hundreds of leagues in length, could have been deposited in the central 



