CORAL REEFS. 385 



application of his views in detail is characterised by great 

 ability ; but we can afford room only for a summary of the 

 few leading points. 



Look at the theory first in its application to the atoll, or 

 simple coral islet with its interior lagoon. Instead of making 

 this lagoon to represent a pre-existing cavity of the same 

 form, as in the volcanic hypothesis, Mr. Darwin considers 

 that it designates the place once occupied by a point of land 

 more or less elevated, which has subsided downwards into the 

 sea, leaving the coral reef circling round the centre, growing 

 over its surface, and rising upwards by new constructions, 

 where the subsidence has still continued. Let a mountain 

 peak like Tahiti, girt round with a coral reef, sink downwards 

 from subsidence of the submarine area or other cause, and 

 we should have the conditions just described taking place; 

 and representing, by the various aspects of these islands, the 

 stages of change from a lofty mountain to a few low points 

 of land in the lagoon, and then to the simple coral islet 

 barely rising out of the sea. Let the subsidence elsewhere 

 be sudden instead of gradual and we should find sub- 

 merged atolls like the Chagos Bank ; the power of replace- 

 ment upwards being lost by the depth to which the surface 

 has sunk below the sea. 



Next, as to the linear reefs, such as the great barrier front- 

 ing the N. E. Australian continent. We have already spoken 

 of the intimate relation of these to the reefs encircling 

 islands ; and we shall find the conditions of the theory apply- 

 ing to both in the same manner, and with equal probability. 

 Suppose a prolonged line of reef to be built up on the shelf 

 of the coast as it declines into the sea, leaving a narrow 

 channel of water between. If the continental land gradually 

 sinks, the line of coast will recede inwards, leaving a wider 

 channel between it and the reef the latter retaining its 

 position, and being continually replaced upwards by fresh 



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