22 HISTORY OP BRITISH ZOOPHYTES. 



the level of the sea, and let its diameter at the base be 

 twenty miles. Let us suppose that it has reached its ulti- 

 matum of upheaval, that all for a time is stable. Around 

 its shores our little active polypes will begin their opera- 

 tions, and in the course of time will form around the island 

 a fringing coral reef, the breadth of which will be consi- 

 derable if the shore, instead of being precipitous, is of a 

 shelving nature. Let us suppose, that when the fringing 

 reef is formed, the island begins to be depressed, to sink 

 again into the deep, and that this submergence for a 

 a lengthened period slowly but gradually goes on. The 

 fringing reef, however, goes down along with it ; and were 

 the polypes to raise their reef no higher they would soon 

 perish, for it has been ascertained that they cannot live 

 in a great depth of water. But it is their delight to 

 work, and their instinct to carry on their operations up- 

 wards; so that while the island is descending into the 

 depths, their reef is ascending. Let us suppose that the 

 island is conical, and that its diameter, which was twenty 

 miles at the base, is only ten miles half-way to the summit ; 

 and let us further suppose that the submergence has pro- 

 ceeded till the island, which was originally 6000 feet in 

 height, is only 3000, and its diameter at the water's edge 



