298 CORALS AND CORAL ISLANDS. 
boring animals may riddle the coral with holes or tubes. 
But generally the erosion is superficial; the large masses 
referred to showed little of it. Such dead surfaces in corals 
are generally 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. 
e. Every stream, says 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 fath- 
oms (270 feet) deep. with an outlet of much less depth could 
not be deepened by such means nor protect itself from shallow- 
ing. 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 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 there- 
fore 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 

