CORALS AND CORAL REEFS — VAUGHAN. 



223 



Murray 1 in 1880 published the following summary of his opinions 

 on the formation of coral reefs: 



That when coral plantations build up from submarine banks they assume an 

 atoll form, owing to the more abundant supply of food to the outer margin, 



Fig. 6. — Copy of Darwin's figure illustrating conversion of a fringing 

 into a barrier reef, according to his hypothesis. aa. outer edge of 

 the reef at the level of the sea. bb. shores of the island. a'a'. 

 Outer edge of the reef, after its upward growth during a period of sub- 

 sidence. CC. The lagoon-channel between the reef and the shores of 



THE NOW ENCIRCLED LAND. B'B'. THE SHORES OF THE ENCIRCLED LAND. 



N. B. — In this and the following cut the subsidence of the land could only 

 be represented by an apparent rise in the level of the sea. 



and the removal of dead coral rock from the interior portion by currents and 

 by the action of the carbonic-acid gas dissolved in sea water. 



That the barrier reefs have been built out from the shore on a foundation of 

 volcanic debris or on a talus of coral blocks, coral sediment, and pelagic shells, 

 and the lagoon channel is formed in the same way as a lagoon. 



jl 4*__C_ . A 



Fig. 7. — Copy of Darwin's figure illustrating conversion of a bar- 

 bier REEF INTO AN ATOLL, ACCORDING TO HIS HYPOTHESIS. A'A'. OUTER 

 EDGES OF THE BARRIER REEF AT THE LEVEL OF THE SEA. THE COCOANUT 

 TREES REPRESENT CORAL ISLETS FORMED ON THE REEF. CC. THE LAGOON 

 CHANNEL. B'B'. THE SHORES OF THE ISLAND, GENERALLY FORMED OF 

 LOW ALLUVIAL LAND AND OF CORAL DETRITUS FROM THE LAGOON CHAN- 

 NEL. A" A". The outer edges of the beef, now forming an atoll. 

 C. The lagoon of the newly formed atoll. According to the 



SCALE, THE DEPTH OF THE LAGOON AND OF THE LAGOON CHANNEL IS EX- 

 AGGERATED. 



That it is not necessary to call in subsidence to explain any of the char- 

 acteristic features of barrier reefs or atolls and that all these features would 

 exist alike in areas of slow elevation, of rest, or of slow subsidence. 



Alexander Agassiz and Stanley Gardiner were in essential accord 

 with the opinions of Semper and Murray. 



1 Murray, John, On the structure and origin of coral reefs and islands : Roy, Soc. 

 Edinburgh Proc, vol. 10, 1879-1880, pp. 505-518, 1880. 



