CORAL REEFS 39 I 



depths — to depths, in fact, at which reef- forming corals do not 

 live. 



It seems ohvious, tlierefore, that tlie atolls and barrier-reefs 

 are resting upon some stratum which could not possibly have 

 been formed by reef-building organisms at the same relative 

 position it has now, and the questions arose. What is the sub- 

 stratum and how was it formed ? 



If this stratum is a coral rock, it is clear that it must have 

 been formed at a time when it was nearer to the surface of the 

 sea than it is now, and that it must have subsided subsequently 

 to greater depths. If, on the other hand, it is a primitive rock, 

 we must assume that in such regions as the Indian Ocean and 

 the South Pacific, where the archipelagoes of atolls extend for 

 hundreds of miles, there are chains of mountain ranges with 

 peaks reaching to a uniform level beneath the surface of the sea. 

 " But we cannot believe that a broad mountain summit Jies buried 

 at the depth of a few fathoms beneath every atoll, and nevertheless 

 that throughout the immense areas above named not one point 

 of rock projects above the level of the sea. Tor we may judge 

 of mountains beneath the sea by those on land, and where can 

 we find a single chain, much less several such chains many 

 hundred miles in length, and of considerable breadth, with broad 

 summits attaining the same height from within 120 to 180 feet ? " ^ 



To account for the observed facts of the atolls and barrier- 

 reefs, Darwin conceived and expounded the subsidence theory. 

 According to this theory, the regions where atolls now occur were 

 at one time dry land, or an archipelago of volcanic islands 

 surrounded by fringing reefs of the ordinary type. A gradual 

 subsidence of the land took place, and the area of the land 

 diminished ; but the area enclosed by the coral reefs did not 

 diminish in a corresponding degree, and the young corals growing 

 on the debris of the older ones as they sank continued the 

 growth of the reef in a direction nearly vertical to the sea- 

 bottom. The fringing reefs thus became barrier reefs, and they 

 were separated from the land by a lagoon of considerable depth. 

 Finally, when the mountain peaks disappeared beneath the 

 waves, a ring-shaped reef or atoll was all that was left to mark 

 the position of the former land. 



The fundamental assumption in the subsidence-theory is that 

 ' C. Darwin, Coral Reefs, 3rd edition, 1889, p. 125. 



