1 82 CORAL-REEFS. 



only with respect to the few last hundred feet of rise. But 

 with reference to the whole vast amount of subsidence, 

 necessary to have produced the many atolls widely scattered 

 over immense spaces, it has already been shown (and it is, 

 perhaps, the most interesting conclusion in this volume), 

 that the movements must either have been uniform and 

 exceedingly slow, or have been effected by small steps, 

 separated from each other by long intervals of time, during 

 which the reef-constructing polypifers were able to bring 

 up their solid frameworks to the surface. We have little 

 means of judging whether many considerable oscillations of 

 level have generally occurred during the elevation of large 

 tracts ; but we know, from clear geological evidence, that 

 this has frequently taken place ; and we have seen on our 

 map, that some of the same islands have both subsided and 

 been upraised. I conclude, however, that most of the 

 large blue spaces have subsided without many and great 

 elevatory oscillations, because only a few upraised atolls 

 have been observed : the supposition that such elevations 

 have taken place, but that the upraised parts have been 

 worn down by the surf, and thus have escaped observation, 

 is overruled by the very considerable depth of the lagoons 

 of all the larger atolls ; for this could not have been the 

 case, if they had suffered repeated elevations and abrasion. 

 From the comparative observations made in these latter 

 pages, we may finally conclude, that the subterranean 

 changes which have caused some large areas to rise, and 

 others to subside, have acted in a very similar manner. 



Recapitulation. — In the three first chapters, the principal 

 kinds of coral-reefs were described in detail, and they 

 were found to differ little, as far as relates to the actual 

 surface of the reef. An atoll differs from an encircling 

 barrier-reef onlv in the absence of land within its central 



