MUSEUM OF COMPARATIVE ZOOLOGY. 125 



fringing reef was formed.* There was a cap of coral limestone over 

 the whole island ; the very top of the island, twelve hundred feet high, 

 being a block of worn and undermined coralline limestone, then tiers of 

 cliffs intervening between the top and existing sea cliffs. " Christmas 

 Island thus appearing to be a remarkable instance of the complete casing 

 with coral of an island, which, from the time that its first nucleus came 

 within the reef-building zone, has been steadily subjected to a movement 

 of upheaval varied by pauses, during which the cliffs were eroded by the 

 sea." 



That the volcanic nucleus has not been exposed is undoubtedly due, 

 as has been suggested by Guppy,^ to the fact that the upheaved island 

 has not been exposed to denuding agencies for a sufficiently long period 

 of time. 



Murray,^ who had unusually good opportunities for examining nu- 

 merous coral reefs of the Pacific, published a remarkable paper on the 

 formation of reefs in the Proceedings of the Royal Society of Edin- 

 burgh, in which he gives an explanation of the formation of channels 

 between barrier reefs and the mainland band, and of the lagoons of 

 atolls, based upon the solvent action of sea-water saturated with car- 

 bonic acid upon coral limestone. That this solvent action is a powerful 

 factor in corroding the surface of coral reef, and carrying off surplus 

 limestone in solution is not to be denied, but to consider it the principal 

 cause of the formation of lagoons and of channels between barrier reefs 

 is perhaps pressing the theory too far. It undoubtedly has acted in 

 many cases powerfully enough to corrode the whole surface of reefs ex- 

 posed to action of water so saturated with carbonic acid. I would re- 

 fer specially to the surface of reefs like the fringing reefs of Honolulu, 

 the corroded breccia reef rock found at many points of the Keys of 

 Florida, and the evidence of the same action to be found on the shore 

 deposits of coral along the whole northern coast of Cuba, where the 

 shore reef exists parallel with the great Cuban barrier reef. Similar 

 action, of course, is taking place constantly in limestone districts, through 

 which waters saturated with carbonic acid percolate, forming caves and 

 other cavities so characteristic of these formations. 



* A similar condition of things exists at Barbados, where tlie volcanic reef centre 

 crops out only on the summit of the island, and the sides of the cone are covered 

 with coral reef terraces, which are one of the most characteristic features of the 

 island as seen from the sea. A. Agassiz, Three Cruises of the Blake, figs. 39, 46. 



2 Nature, January 5, 1887, p. 223. 



8 Murray, John, Proc. Royal See. Edinb., 1879-80, p. 505. 



