Xll 



PRE FA TOR Y NOTE. 



in our sixth chapter) that when the land is prolonged beneath 

 the sea in an extremely steep slope, reefs formed there during 

 subsidence will remain closely attached to the shore, and will 

 remain undistinguishable from fringing-reefs. Now we know 

 that the submarine flanks of most atolls are very steep; and if 



Fig. 5. —Madrepore (Goniopora columna, Dana). Natural size. 



an atoll after upheaval and before the sea had eaten deeply 

 into the land, and had formed a broad flat surface, were again 

 to subside, the reefs which grew to the surface during the sub- 

 siding movement would still closely skirt the coast.' In this con- 

 nection, Darwin's letter to the same observer is also interesting 

 and instructive ; he had received from Professor Semper the 



