XI 



The Bed of the Oceans 



211 



corals had built in that layer, but that the foundations had 

 been gradually sinking. Thus he supposed a fringing reef 

 (I, Fig. 40) to form round a volcanic island, and as the 

 island slowly subsided the corals built the reef higher and 

 higher, keeping pace with 

 the subsidence. In time, 

 as the outer edge of the 

 coral grows fastest on 

 account of the greater 

 abundance of oxygen in 

 the breakers, the reef 

 would widen and grow 

 higher seaward, forming a 

 barrier reef by the time 

 subsidence has brought 

 the sea-level to the posi- 

 tion (II). Finally, sub- 

 sidence submerges the 

 whole mountain below 

 the surface, and the barrier 



FIG. 40. Darwin's theory of the origin of 

 Coral Islands. I, II, and III show suc- 

 cessive levels of the sea brought about 

 by subsidence of a volcanic island (solid 

 black). The corresponding coral forma- 

 tions, respectively fringing reef, barrier 

 reef, and atoll, are shown by ^different 

 shading. 



reef grows up to form an 

 atoll (III) peeping above sea-level. In recent years a num- 

 ber of objections to this widely accepted theory have been- 

 made by many investigators. It has been pointed out that 

 atolls are as common in areas which are being gradually 

 elevated as in those that are subsiding. Dr. Guppy, in the 

 course of a study of the Solomon Islands, where many reefs 

 have been elevated far above sea-level, also found that the 

 coral limestone is never of greater thickness than about 120 

 feet, and he thus casts doubt on the existence of vast sub- 

 merged walls of coral. He found that the cake of coral rock 

 rested either on volcanic rock or on rocks formed by the 

 consolidation of pteropod or globigerina ooze. 



282. Murray's Coral Island Theory. During the cruise 

 of the Challenger Dr. John Murray formed another theory, 

 which has been strikingly confirmed by the observa- 

 tions of Dr. Guppy and others. He believes that the 

 foundation for coral reefs is in every case supplied by 

 submarine peaks. Some of these may have been formed 



