ANNOTATIONS AND ADDITIONS. 77 



crater, is opposed by their great dimensions, the diameters 

 of many of them being 30, 40, and sometimes even 60 

 geographical miles. Our fire-emitting mountains have no 

 such craters ; and if we would compare the lagoon, with its 

 submerged interior and narrow enclosing reef, to one of the 

 annular mountains of the moon, we must not forget that 

 those lunar mountains are not volcanoes, but wall-surrounded 

 districts. According to Darwin, the process of formation 

 is the following : He supposes a mountainous island sur- 

 rounded by a coral-reef, (a " fringing reef" attached to the 

 shore), to undergo subsidence: the "fringing reef" which 

 subsides with the island is continually restored to its level 

 by the tendency of the coral-animals to regain the surface 

 of the sea, and becomes thus, as the island gradually sinks 

 and is reduced in size, first an " encircling reef" at some 

 distance from the included islet, and subsequently, when 

 the latter has entirely disappeared, an atoll. According to 

 this view, in which islands are regarded as the culminating 

 points of a submerged land, the relative positions of the 

 different coral islands would disclose to us that which we 

 could hardly learn by the sounding line, concerning the con- 

 figuration of the land which was above the surface of the sea 

 at an earlier epoch. The entire elucidation of this attractive 

 subject, (to the connection of which with the migrations of 

 plants and the diffusion of races of men attention was 

 called at the commencement of the present note), can only 

 be hoped for when inquirers shall have succeeded in ob- 

 taining greater knowledge than is now possessed of the 

 depth and the nature of the rocks on which the lowest 

 strata of the dead corals rest. 



