124 



POPULAR ILLUSTRATIONS OP 



have been unchallenged for the last twenty-five years. Now upon 

 the above data, of which I will say more presently, Mr. Darwin 

 accounts for reef formations"thus : 



Suppose a (Fig. 173) to be a section of a mountain portion of high 

 land, which is in a state of temporary quiescence, neither subsiding 

 nor rising, and that b is a coral reef, which has gradually been 

 formed by our friends the polyps. Now let us suppose, what is in 

 fact a frequent occurrence in the Pacific, that an active volcano 

 opens in the summit of a, as seen in the sectional diagram at 

 Fig. 174. Simultaneously with this active volcanic action, it accords 



Fig. 173. 



Fig. 174. 



with experience that the mountain would begin to subside, and as it 

 did so the coral reef begun at b (Fig. 153), would continue to rise 

 as the polyps which only live from twelve to thirty fathoms deep, 

 and never above low water would keep adding to their wall just in 

 proportion as the land sank. Now the polyps for many reasons 

 thrive best outside the reef. They are, in fact, every now and then 

 destroyed between the inner boundary of the reef and the shore, and 

 the consequence is that the outside of the reef gets to be the highest, 

 and what is called a lagoon channel is formed between this front 

 and the shore, as seen in Fig. 174 (c). This lagoon is more or less 



