THE PACIFIC OCEAN. 273 



these animal secretions must have been deposited 

 within that distance from the surface. At the same 

 time, it is no less true that the water in the immediate 

 vicinity of the islands is fathomless, and that the 

 descent of their outer edge is remarkably abrupt 

 and precipitous. The only satisfactory explanation of 

 the phenomenon appears to be the one proposed and 

 ably supported by Mr. Darwin, in his elaborate 

 treatise on Coral reefs. Many islands of the com- 

 mon rock formation are found in the Pacific, on the 

 shelving sides of which, a few fathoms below water, 

 the coral animals have fixed their stony habitations, 

 forming what is called a fringing reef, distinguished 

 from others by being immediately attached to the 

 land, without the intervention of any lagoon or 

 channel of water. Mr. Darwin supposes that every 

 island in the Pacific originally presented this struc- 

 ture, but that wherever a variation at present exists, 

 the solid rock has been gradually, and perhaps very 

 slowly, subsiding to a lower level. Now, let us 

 assume this state of things for a moment, and look at 

 the results. We must, however, mention two well- 

 ascertained instincts of the Polype : the one is, that 

 it works up towards the light ; the other, that its 

 proceedings are most vigorous at the outer edge, 

 where it is washed by the beating waves. Let A 

 represent the section of a rocky island; B, B, the level 

 of low- water; and D, the reef of coral frimnnor the 

 coast. After the lapse of time, during which it lias 

 been subsiding, the water-level stands at />, b; the 

 coral at D has died from the too great depth, but the 

 animals have been working upwards upon the dead 



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