ORIGIN OF THE BARRIER REEF. 225 



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 currents within, while 

 the inner reefs contribute a large proportion 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 accumulations 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 increases one foot in height in a century, the 

 inner reef, according to this supposition, would increase but 

 half a foot ; and any rate of subsidence between the two men- 

 tioned would sink the inner reefs more rapidly than they could 

 grow, and cause them to disappear. There is therefore not 

 only no objection \o the theory from the existence of wide 

 channels and open seas ; on the contrary, their non-existence 

 is incompatible with the mode of action going on. They 

 afford the strongest support to the theory. 



A wide, flat reef, continuous over extensive reef-grounds, 



could be formed only upon a nearly level bank, where there were 



consequently no hills to pour in detritus and otherwise retard 



growth over the interior portions ; and even then it would be 



liable to be cut up by the action of currents, destroying growing 



corals over its interior parts. 



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