14 Proceedings of the Royal Physical Society. 



sonal observation and a comparison of the Admiralty charts, 

 that most exaggerated notions prevail regarding the depth 

 of water immediately outside the reef, which is usually sup- 

 posed to be very great. After minutely exploring the barrier- 

 reef of Tahiti, and sounding the water both inside and outside 

 the reef, he found that the slopes are just such as might be 

 looked for, on the supposition that the corals have grown 

 up without any sinking of the bottom. The accompanying 

 section (Fig. 1), drawn on a true scale, will show that there 

 is nothing abnormal in the declivities. Beginning near the 

 shore, or wherever the bottom, whether of rock or sediment, 

 comes within the range of the reef-builders, a barrier-reef 

 grows vigorously along its outer face, while its inner parts, 

 as in the case of an atoll, and for the same reason, are 

 enfeebled and die. The force of the breakers tears off huge 

 masses, sometimes 20 or 30 feet long, from the face of the 

 reef, especially where, from the borings of moUusks, sponges, 

 etc., the coral-rock has been weakened. These blocks tumble 

 down the seaward face of the reef, forming a remarkably 

 steep talus. It is this precipitous part of the reef which has 

 probably given rise to the notion that the water outside sud- 

 denly descends to a profound depth. The steep front of 

 fallen blocks is succeeded by a declivity covered with coral- 

 sand, beyond which the bottom slopes away at an angle of 

 no more than 6°, and is covered chiefly with volcanic detritus. 

 Mr Murray insists that any seaward extension of the reef 

 must be on the summit of the talus of broken coral. The 

 reef will gradually recede from the shore of the island or 

 continent, and will leave behind, here and there, a remnant 

 to form an island in the slowly -broadening lagoon channel. 



The very general occurrence of proofs of elevation among 

 the regions of barrier-reefs and atolls is in harmony with the 

 volcanic origin of the ground on which these coral formations 

 have grown, but, as Mr Murray contends, is most difficult of 

 explanation on the theory of subsidence. He affirms that 

 all the chief features of coral-reefs and islands not only do 

 not necessarily demand the hypothesis of subsidence, but 

 even in areas where the movement is an upward one, may be 

 satisfactorily accounted for by the vigorous outward growth 



