ON CORAL AND CORAL REEFS 123 



are no coral reefs upon the west coast of America, nor 

 upon the west coast of Africa ; and it is a general fact 

 that the reefs are interrupted, or absent, opposite the 

 mouths of great rivers. The causes of this apparent 

 caprice in the distribution of coral reefs are not far to 

 seek. The polypes which fabricate them require for 

 their vigorous growth a temperature which must not 

 fall below 68 Fahrenheit all the year round, and this 

 temperature is only to be found within the distance on 

 each side of the equator which has been mentioned, or 

 thereabouts. But even within the coral zone this degree 

 of warmth is not everywhere to be had. On the west 

 coast of America, and on the corresponding coast of 

 Africa, the currents of cold water from the icy regions 

 which surround the South Pole set northward, and it 

 appears to be due to their cooling influence that the sea 

 in these regions is free from the reef builders. Again, 

 the coral polypes cannot live in water which is rendered 

 brackish by floods from the land, or which is perturbed 

 by mud from the same source, and hence it is that they 

 cease to exist opposite the mouths of rivers, which 

 damage them in both these ways. 



Such is the general distribution of the reef-building 

 corals, but there are some very interesting and singular 

 circumstances to be observed in the conformation of 

 the reefs, when we consider them individually. The 

 reefs, in fact, are of three different kinds ; some of them 

 stretch out from the shore, almost like a prolongation 

 of the beach, covered only by shallow water, and in the 

 case of an island, surrounding it like a fringe of no con- 

 siderable breadth. These are termed "fringing reefs." 

 Others are separated by a channel which may attain a 

 width of many miles, and a depth of twenty or thirty 

 fathoms or more, from the nearest land ; and when this 



