XX INTRODUCTION. 



sink, when its lowest point has reached the level of the sea. This break 

 would thus form an entrance to the lagoon, much as is formed the entrance 

 to any lagoon or sound. Should this mass be elevated a second, a third, 

 or a fourth time, we may find one, or two, or more entrances to the old 

 lagoons and sounds according to the rate of denudation and of erosion of 

 the elevated mass during the periods of rest. 



Tlie coralliferous limestone rings would be fairly continuous in case of 

 a slow rate of denudation and erosion ; if broken through by the action 

 of the sea and with a rapid rate of denudation, only disconnected patches, 

 more or less numerous, according to the rate of erosion, would indicate the 

 former ring ; and finally, with a very rapid denudation and erosion, both of 

 tlie exterior and interior face of the lagoon, or sound, nothing would be 

 left of the elevated mass except the submerged reef-ring. This becomes 

 still more complicated when the limestone mass, while elevated, has been 

 broken through by the underlying volcanic rocks, and they have displaced 

 portions of the coralliferous limestone beds and left them more exposed 

 to the effects of denudation and of erosion, especially when this action has 

 taken place on the outer face of the elevated mass, and left cuts and 

 openings connecting the outer sea and the interior basins, which would 

 thus soon be transformed into great sounds or lagoons. The erosive as 

 well as the solvent action of the sea would soon level the outer rim to 

 the plane of the sea, the further disintegration being stopped by the growth 

 of recent corals or of coralline Alga3 upon the surface of the coralliferous 

 or massive limestone eroded to the level of the sea or below it.^ With 

 the more rapid erosion and denudation, both atmospheric and marine, the 

 limestones would rapidly disappear, and there would be left only the 

 volcanic mass which had uplifted them, with here and there a remnant of 

 the limestones to indicate the probable course of events. Of course, when 

 the volcanic masses come up without elevating any limestone beds the 

 conditions are much simpler, and it becomes only a question of the 



^ According to Dana's views the existence of such foundations " only prove that, in coral seas, 

 corals will grow over any basis of rock that may offer where the water is right in depth, and do not 

 nullify any of the evidence of subsidence" (Am. Jour. Scien., XXX., September, 1885, p. 181); yet 

 these very instances are advanced as supporting the theory of subsidence, and we may justly reply that 

 they have great interest as they arc opposed to the application of the theory of subsidence as explana- 

 tory of the formation of coral reefs in the areas where these rock bases occur. 



