THE VOYAGE OF THE BEAGLE 493 



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 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 fath- 

 oms. Now there are enormous areas in the Pacific and Indian 

 Ocean, 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 

 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 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 profoundest 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, innumerable 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 

 surface of the globe can we find a single chain of mountains, 

 even a few hundred miles in length, with their many sum- 

 mits 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 



