284 TOILERS IN THE SEA. 



its less soluble ingredients are being deposited in the 

 form of coral sand. There are thousands of other 

 cases where the lagoon interval, between the front of 

 the reef and the shores, has been so far widened that 

 it is taking the form of a barrier, as distinguished 

 from a fringing reef, and where the lagoon can be 

 navigated by small boats. But when we come to 

 the larger atolls, and the great seas included between 

 a barrier reef and its related shores, the mind may 

 well be staggered by the enormous quantity of 

 matter, which it is suggested has been dissolved, 

 removed, and washed away. The breadth of the 

 sheltered seas, between barrier reefs and the shore, is 

 measured in some cases not by yards, nor hundreds of 

 yards, not by miles, but by tens of miles, and this 

 breadth is carried on in linear directions, not for 

 hundreds of miles, but for thousands. And yet 

 there is one familiar idea in geology which might 

 have helped Darwin, as it is much needed to help us, 

 even now, to conceive it. It is the old doctrine of 

 the science, long ago formulated by Hutton, that the 

 work of erosion, and of denudation, must be equal to 

 the work of deposition. Rocks have been formed 

 out of the ruins of older rocks, and those older rocks 

 must have been worn down, and carried off, to an 

 equivalent amount. So it is here, with another kind 

 of erosion, and another kind of deposition. The 

 coral building animals can only get their materials 



