38 



NATURAL HISTORY. 



The edge of the reef would continue its upward growth, and so keep at 

 the surface, as fast as its foundation sank, but between the edge and the 

 shore a strip of clear water would be formed, because the coral here would 

 not be properly nourished, and would also be smothered by the debris 

 broken by the waves from the edge of the reef. In this way the former 

 '•fringing reef " would be converted into a "barrier reef." If the subsi- 

 dence still continue, at last there will be a circular reef containing a central 

 lake or lagoon, a type of island to which the name atoll (a Polynesian 

 word) has been applied. True atolls, however, are rare. Usually the 

 island has a gap on one side, through which entrance may be had to the 

 lagoon. This is clue to the fact that the prevailing winds and currents are 

 almost constantly in one direction, and that the windward side thus is 

 better fed, and hence grows more rapidly than that to the leeward, and 

 the entrance to the lagoon is always to the leeward side of the atoll. 



Fig. 40. — A circular coral island (atoll), with a lagoon in the centre. 



This matter of wind and current has lately been thought to be a very 

 important element in the determination of the shape of coral islands, and 

 in the case of the Florida Keys this certainly is the case. If we examine 

 the most of these islands, we see that they are all long, and that their 

 longer axis is parallel to the principal current which passes them. At first 

 sight it would seem that there were one or two exceptions to this, a few 

 having their longer axes at right angles to the general direction of the 

 Gulf Stream, but in these cases it is found that there are secondary currents 

 flowing down the western coast of Florida, and that these have modified 

 the islands in question. 



Besides the mere growth of the coral, there are other elements to con- 

 sider in the growth of a coral- island. A coral reef is constantly dying, 

 and the dead branches are broken into fragments and ground into sand 

 by the waves. Some of this sand is of course carried out to sea, but much 



