Sect. II. ATOLLS. 38 



that the exterior angle is much Yaried. We can, 

 however, discern the effects of uniform laws in the 

 close similarity in form between the sections of Keel- 

 ing atoll and of the atolls in the Low Archipelago, 

 — in the general steepness of the reefs of the Maldiva 

 and Chagos atolls, — and in the perpendicularity of 

 those rising out of water always tranquil ; but from 

 the complex action of the surf and currents on the 

 growing powers of the coral and on the deposition 

 of sediment, we can by no means follow out all the 

 results. 



Where islets have been formed on the reef, that 

 part which I have called the ' flat,' and which is partly 

 dry at low water, appears similar in every atoll. In 

 the Marshall group in the N. Pacific, it may be 

 inferred from Chamisso's description, that the reef, 

 where islets have -not been formed on it, slopes gently 

 from the external margin to the shores of the la- 

 goon : Flinders states that the Australian barrier has 

 a similar inclination inwards, and I have no doubt 

 it is of general occurrence, although, according to 

 Ehrenberg, the reefs of the Red Sea offer an exception. 

 Chamisso observes that ' the red colour of the reef (at 

 the Marshall atolls) under the breakers is caused by a 

 Nullipora, which covers the stone wherever the waves 

 beat ; and, under favourable circumstances, assumes a 

 stalactitical form,' — a description perfectly applicable 

 to the margin of Keeling atoll. 1 Although Chamisso 



1 Kotzebue's "First Voyage, vol. iii. p. 142. Near Porto Praya, in 

 the Cape de Verd Islands, some basaltic rocks, lashed by no inconsider- 



I) 



