CORAL REEFS. APPENDIX. 223 



H. B. Guppy. " Preliminary Note on Keeling Atoll, known also as the Cocos Islands." 



Nature, Jan. 3, 1889. 



In this paper (letter addressed to Mr. Murray) the author 

 presents some interesting facts pertaining to the formation of 

 horse-shoe shaped atollons. His conclusion may be briefly 

 stated : " that wherever a coral island stems a constant surface- 

 current, the sand produced by the breakers on the outer edge 

 of the reef will mostly be deposited by the current on each side 

 of the island in the form of two lateral banks or extensions, 

 giving the island ultimately a horse-shoe form, with the con- 

 vexity presented against the current." 



A bank may then be " thrown up across the mouth of the 

 horse-shoe, and a small atoll with a shallow lagoonlet is pro- 

 duced." Other points reached in Mr. Guppy's examination of 

 the Keeling atoll are that "the lagoon is rapidly filling up 

 with sand and coral " and "that the outward extension of the 

 reef is effected, not so much by the seaward growth of the 

 present edge of the reef, as by the formation outside of it of a 

 line of growing corals, which, when it reaches the surface re- 

 claims, so to speak, the space inside it, which is soon filled up 

 with sand and reef-debris." 



Mr. Guppy prefaces his paper with an appeal to the shallow- 

 ness of lagoons, and criticizes our exaggerated notions of these 

 structures. On a true scale it is claimed that a typical lagoon 

 (i would be represented by a film of water occupying a slight 

 hollow in the level mountain-top." The author further ex- 

 presses himself as follows : " By thus grasping these facts, we 

 at once perceive that by reason of our failing to view an atoll 

 in relation to its surroundings, and through our misconcep- 

 tions of its dimensions, we have been led to introduce a great 

 cause to explain a very small effect. The slightly raised mar- 

 gins can be easily explained by causes dwelt upon by Murray, 

 Agassiz, and others. No movement of the earth's crust is 

 necessary for this purpose. The mode of growth of corals, the 

 action of the waves, and the influence of the currents, afford 

 agencies quite sufficient to produce the slightly raised margins 



