348 THE BOTTOM OF THE SEA. 



more than 1200 miles. Each year lias furnished, 

 and still furnishes, proofs of the disappearance or 

 diminished size of islands in tliis immense zone. 



It has been seen in a preceding chapter how the 

 incessant labours of the coral insects compensate for 

 the sinking of the soil, and how the rapidity of the 

 growth of the coral reefs furnishes a measure of the 

 rate of subsidence. The size of the reefs also indi- 

 cates the epochs since which the movement of the 

 soil has continued. The annual growth in height of 

 the polypiers is 0"'*003. Now certain reefs are several 

 hundred yards in depth. Those of the Fiji Islands, 

 for example, are about 1000 yards, those of the 

 Gambier Islands about 400, and those of Tahiti about 

 80 yards. If the growth of those reefs has always 

 been at the same rate, 300,000 years have been neces- 

 sary for the production of the reefs of the Fiji Islands. 



The coral insects grow only near the surface of 

 the water : they have therefore sunk 1000 yards at 

 the Fiji Islands since they first commenced their 

 work, and that has been during a period of 300,000 

 years. 



The second region of subsidence comprehends New 

 Caledonia, Australia, and the basin of the' Indian 

 Ocean, including the atolls* of the Chagos Bank and 



* An atoll is a circular wall or reef of coral enclosing the sea, 

 within which it resembles a small lake. — Tb. 



