220 CORALS AND CORAL LSLANDS. 



miles in diameter, and that twenty and thirty miles should be 

 a common size. Facts give no support to such an assump- 

 tion. 



d. It supposes that the high islands of the Pacific, in the 

 vicinity of the coral islands, abound in craters ; while, on the 

 contrary, there are none, so far as is known, in the Marquesas, 

 Gambier, or Society Group, the three which lie nearest to the 

 Paumotus. Even this supposition fails, therefore, of giving 

 plausibility to the crater hypothesis. 



Thus at variance with facts, the theory has lost favour, and it 

 is no longer sustained even by those who were once its strongest 

 advocates. 



The question still recurs with regard to the basement of 

 coral islands, and the origin of their lagoon character. Shall 

 we suppose, with some writers, that these islands were planted 

 upon submarine banks, within one hundred and fifty feet of 

 the surface of the sea ? As has been said, there is no authority 

 for the supposition. We nowhere find regions over our con- 

 tinents widi elevations so uniform in height ; and submerged 

 banks of this kind are of extremely rare occurrence. If such 

 patches of submerged land existed, the lagoon structure would 

 still be as inexplicable as ever ; for the growing reefs of the 

 Pacific show that corals may flourish alike over all parts of the 

 bank, where not too deep. The zoophyte can by no means 

 be said to prefer the declivity to the central plateau of the 

 submarine bank ; on the contrary, the part nearest the surface 

 below low-tide level, abounds in the largest species of corals. 



II. ORIGIN OF CHANNELS WITHIN BARRIERS. 



A Study and comparison of the reefs of different kinds, — 

 fringing, barrier and atoll, — throughout the oceans, is the only 

 philosophical mode of arriving at any conclusion on this sub- 

 ject. This course Mr. Darwin has happily and successfully 

 pursued, and has arrived, as we have reason to beheve, at the 

 true theory of barrier reefs and coral islands. It is satisfactory, 

 because it is a simple generalization of facts. The explora- 



