468 AREAS OF SUBSIDENCE. '.chap. xx. 



pelag-oes, 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 ori- 

 ginally 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 depo- 

 sited 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 witliin 20 to 30 

 fathoms, or 120 to 180 feet, of the surface of the sea, and not 

 one single point above tliat 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, wlience 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 of the 

 corals. It is impossible here to enter into all the necessary 

 details, but I venture to defy* any one to explain in any other 

 manner, how it is possible that numerous islands should be dis- 

 tributed throughout vast areas — all the islands being low — all 

 being built of corals, absolutely requiring a foundation within 

 a limited depth from the surface. 



Before explaining how atoll-formed reefs acquire their pecu- 

 liar structure, we must turn to the second great class, namely, 

 Barrier-reefs. These either extend in straight lines in front of 

 the shores of a continent or of a large island, or they encircle 

 smaller islands ; in both cases, being separated from the land by 



* It is remarkable that Mr. Lyell, even in the first Edition of his ' Princi- 

 ples of Geology,' inferred that the amount of subsidence in the Pacific must 

 nave exceeded that of elevation, from the area of land being very small 

 relatively to the agents there tending to form it, namely, the growth of coral 

 and volcanic action. 



