500 EFFECTS OF SLOW SUBSIDENCE. 



upon the land, and increase the distance of its shore from the 

 reef. In this manner the skirtiny reef, upraised from a depth of 

 ten or fifteen fathoms, and at a distance of five hundred yards from 

 the shore, may be gradually converted into a barrier reef, with 

 water one hundred and fifty feet deep on each side of it, and the 

 shore one hundred miles off. Let us imagine the gradual sub- 

 sidence to continue, until the whole of New Holland should be 

 submerged, with the exception of its loftiest hills. These would 

 then remain as rocky islands rising out of the ocean ; but the 

 barrier reef would continue in its present aspect, since it would 

 be still maintained on a level with the surface, by the labours of 

 its innumerable builders, although the depth of its base would be 

 constantly increasing. If a rapid subsidence were to take place, 

 however, the summit would be submerged to a depth inconsistent 

 with the vitality of the Polypes ; and all increase must then 

 cease. 



1144. It is evident that this hypothesis will be equally appli- 

 cable to the case of the lagoon-islands. If the area over which 

 they occur were formerly more elevated, some of its volcanic 

 peaks and circular ridges might have lifted themselves high 

 above the ocean ; others would have been nearer its level ; and 

 others might have been just submerged. Upon the latter, 

 the formation of coral would have begun; and circular reefs 

 would have been built up to the surface. If a slow subsidence 

 then took place, these reefs would still retain their surface-level 

 by addition, whilst a new set of hills would be submerged, and 

 would serve as bases for new coral islets. In this manner 

 all the summits, however different their original elevation, 

 would be rendered of an uniform height ; each, as it was sub- 

 merged, becoming the basis of a coral growth, which keeps pace 

 with the^ progressive lowering of the whole mass ; and so on 

 until all are thus submerged, and no land but coral islets remains 

 above the surface. 



1145. This very ingenious hypothesis corresponds well with 

 the fact that, over certain large areas of the Pacific, we find 

 lagoon islands and barrier reefs abundant, whilst skirting reefs 

 are scarcely ever met with. Moreover, actual proof of a slow 



