Coral Reefs and Islands. 287 



through abrasion and transportation is null or nearly so below 

 the level of its outlet. A basin or channel 45 fathoms (270 

 feet) deep, with an outlet of much less depth, could not be 

 deepened by such means or protect itself from shallowing. 

 The depth of the outlets is not stated, except that they are 

 said to be ship-channels. Moreover, with a tufa bottom, 

 solution could not contribute to the removal, since carbonated 

 waters, although decomposing the tufa, dissolve very little of 

 its ingredients. An elevation in progress would result in 

 making of the channel a closed lake, and finally dry land. 



For the same reason, the small atoll, Kriangle, having, as 

 described, a closed lagoon, could have no deepening of the 

 lagoon from abrasion by tidal currents or wave-action during 

 the progress of an elevation. And if a lagoon have an out- 

 let, the rapid current of the ebb would be confined to the 

 narrow passage-way and a portion of the bottom near it ; 

 through the larger part of the lagoon, as in any other lake, 

 the waters would have scarcely perceptible motion, and 

 therefore slight energy for any kind of work. Hence a 

 lagoon would lose very little by this means, and shallowing 

 would go on unless there were great loss through the solvent 

 action of the waters. An elevation would only hurry the 

 shallowing and end in emptying the lagoon. 



e. Erosion through solvent action is promoted by the 

 presence in the waters both of carbonic acid and organic 

 acids. The material within reach of the tides or waves ex- 

 posed to this action is dead corals and shells, or their debris, 

 and bare coral rocks, occurring over — (1) the outer region of 

 living corals and for a mile or so outside ; (2) the shore 

 platform and the reef, bare at low tide, on which there is 

 comparatively little living coral ; and (3) the lagoon basin. 

 There is nothing in the material within the lagoon to favour 

 solution more than in either of the other two regions ; in fact, 

 the platform and bare reef are most exposed to the action 

 because of the small amount of living corals over them. The 

 outside waters take up what they can through the carbonic 

 acid they contain, and supply thereby the wants of the lime- 

 secreting polyps, shells, &c, and carry on the process of 

 solidification in the debris ; the same waters move on over 

 the atoll reef and take up more lime as far as the acid ingre- 

 dient is present ; and then they pass to the lagoon for work 

 similar to that outside, with probably a diminished amount of 

 free carbonic acid, on account of the loss over the reef-ground 

 previously traversed. 



The lagoon basin is not, therefore, the part of the atoll that 

 loses most bv solution, any more than by abrasion and trans- 



X2 



