186 J. D. Dana — Origin of Coral Reefs and Islands. 



to showed little of it. Such dead surfaces in corals are gener- 

 ally protected by a covering of nullipores and other incrusting 

 forms of life, and the crusts usually spread over the surfaces 

 'pari passu with the dying of the polyps. 



d. Every stream, saj^s Mr. Semper (when explaining, as cited 

 on a preceding page, the origin of the deep channel of the large 

 Pelew island, whose depth is " 35 to 45 fathoms "), " has a 

 natural tendency to deepen its bed." But there is a limit to 

 this action. The eroding or deepening power of a stream 

 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 ingre- 

 dients. 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 outlet, 

 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 eleva- 

 tion would only hurry the shallowing and end in emptying the 

 lagoon. 



e. Erosion through solvent action is promoted by the pres- 

 ence in the waters both of carbonic acid and organic acids. 

 The material within reach of the tides or waves exposed 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 favor 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, etc., and carry on the process 

 of solidification in the debris ; the same waters move on over 



