262 CORALS AND CORAL ISLANDS. 



even open seas within, as in Nanuku and the Exploring Isles, ' 

 are therefore to be received as results of the subsidence, for 

 which explanations should be sought. 



These explanations are at hand, and accord so exactly with 

 facts ascertained, that the existence of inner' passages becomes 

 a necessary feature of such islands. It has been shown that 

 the ocean acts an important part in reef-making ; — that the 

 outer reefs, exposed to its action and to its pure waters, grow 

 more rapidly than those within which are under the influence 

 of marine and fresh-water currents and transported detritus. 

 It is obvious, therefore, that the former may retain themselves 

 at the surface, when through a too rapid subsidence the inner 

 patches would disappear. Moreover, after the barrier is once 

 begun it has growing corals on both its inner and outer 

 margins, while a fringing reef grows only on one margin. 

 Again, the detritus of the outer reefs is, to a great extent, 

 thrown back upon itself by the sea without and the cur- 

 rents within, while the inner reefs contribute a large propor- 

 tion of their material to the wide channels between them. 

 These channels, it is true, are filled in part from the outer 

 reefs, but proportionally less from them than from the inner. 

 The extent of reef-grounds within a barrier, raised by accumu- 

 lations at the same time with the reefs, is often fifty times 

 greater than the area of the barrier itself. Owing to these 

 causes the rate of growth of the barrier may be at least twice 

 more rapid than that of ' the inner reefs. If the barrier in- 

 creases one foot in height in a century, the inner reef, ac- 

 cording to this supposition, would increase but half a foot ; and 

 any rate of subsidence between the two mentioned, would sink 

 the inner reefs more rapidly than they could grow, and cause 

 them to disappear. There is therefore not only no objection to 

 the theory from the existence of wide channels and open seas ; 



