22 HISTORY OF 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 
