XX AREAS OF SUBSIDENCE 497 



last the soil was so sterile that nothing sprang from it. From 

 these observations, 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 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 42c 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 the specified height. Again, 

 in the Indian Ocean there is a space of ocean 1500 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 

 20 to 30 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 and pro- 

 foundest parts of the Pacific and Indian Oceans, at an immense 

 distance from any continent, and where the water is perfectly 

 limpid. It is equally improbable that the elevatory forces 

 should have uplifted, throughout the above vast areas, innu- 

 merable great rocky banks within 20 to 30 fathoms, or 120 

 to 180 feet, of the surface of the sea, and not one single point 

 above that level ; for where on the whole face of the globe 

 can we find a single chain of mountains, even a few hundred 

 miles in length, with their many summits rising within a few 

 feet of a given level, and not one pinnacle above it ? If then 

 the foundations, whence the atoll-building corals sprang, were 

 not formed of sediment, and if they were not lifted up to the 

 required level, they must of necessity have subsided into it ; 

 and this at once solves the difficulty. For as mountain after 

 mountain, and island after island, slowly sank beneath the 

 water, fresh bases would be successively afforded for the growth 



2 K 



