ON CORAL AND CORAL REEFS 131 



which the coral embryos might perch. If, on the other 

 hand, the level of the sea were slowly and gradually 

 lowered, it is clear that the parts of its bottom origi- 

 nally beyond the limit of coral growth would gradually 

 be brought within the required distance of the surface, 

 and thus the reef might be indefinitely extended. But 

 this process would give rise neither to an encircling reef 

 nor to an atoll, but to a broad belt of upheaved coral 

 rock, increasing the dimensions of the dry land, and 

 continuous seawards with the fresh fringing reef. 



Suppose, however, that the sea-level rose instead of 

 falling, at the same slow and gradual rate at which we 

 know it to* be rising in some parts of the world, not 

 more, in fact, than a few inches, or, at most, a foot or 

 two, in a hundred years. Then, while the reef would 

 be unable to extend itself seaward, the sea-bottom out- 

 side it being gradually more and more removed from 

 the depth at which the life of the coral polypes is pos- 

 sible, it would be able to grow upwards as fast as the 

 sea rose. But the growth would take place almost ex- 

 clusively around the circumference of the reef, this 

 being the only region in which the coral polypes would 

 find the conditions favourable for their existence. The 

 bottom of the lagoon would be raised, in the main, only 

 by the coral debris and coral mud, formed in the manner 

 already described; consequently, the margins of the 

 reef would rise faster than the bottom, or, in other 

 words, the lagoon would constantly become deeper. 

 And, at the same time, it would gradually increase in 

 breadth; as the rising sea, covering more of the land, 

 would occupy a wider space between the edge of the 

 reef and what remained of the land. Thus the rising 

 sea would eventually convert a large island with a 

 fringing reef into a small island surrounded by an 



