x PRE FA TOR Y NO TE. 



or the north-eastern coast of North America, since these are in 

 the neighbourhood of the mouths of large rivers which bring 

 down a large amount of mud and other suspended matters from 

 the land. 



Reasoning on these facts, and fresh with the evidences of 

 subsidence, obtained by reading and observation on the South 

 American coast, Darwin conceived and nurtured the theory 

 which is set forth in the succeeding pages. Briefl}', this theory- 

 is as follows : — That — as the polypes cannot live below a 

 depth of ioo feet, and are killed by exposure to sunshine 

 and air, and could not therefore have grown upward from 

 those vast depths to which the coral-masses extend — each atoll 

 began as a fringing-reef, then became a barrier-reef, and at last 

 appeared as a ring of coral with a central lagoon, owing to 

 a slow but progressive subsidence of the site on which the 

 polypes first began to build. If, on this view, a fringing-reef be 

 formed round an island (Fig. 4, 1st period) between the sea- 

 level and the 20-fathom line, and then the island gradually 



Fig. 4.— Illustrating Darwin's theory of formation of the three kinds of Coral 

 Reefs. 



First period, the Fringing-Reef ; second period, the Barrier-Reef; third 

 period, the Atoll. 



sink deeper into the sea, it (z.e., the island) will have become 

 smaller, and the channel between it and the reef wider ; the 

 fringing-reef will in time have become changed into a barrier- 

 reef (Fig. 4, 2nd period), provided that the polypes grow 

 upward at a rate which keeps pace with the depression. Again, 

 another gradual subsidence of the island taking place, and the 

 coral growing upward as fast as the base sinks downward, 

 there would at last result a more or less ring-shaped reef 



