vi.] ON CORAL AND CORAL REEFS. 119 



where to be had. On the west coast of America, and on 

 the corresponding coast of Africa, 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 considerable 

 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 land is an 

 island, the reef surrounds it like a low wall, and the sea 

 between the reef and the land is, as it were, a moat 

 inside this wall. Such reefs as these are called <( en- 

 circling " when they surround an island ; and " barrier " 

 reefs, when they stretch parallel with the coast of a con- 

 tinent. In both these cases there is ordinary dry land 

 inside the reef, and separated from it only by a narrower 

 or a wider, a shallower or a deeper, space of sea, which 

 is called a " lagoon," or " inner passage." But there is a 

 third kind of reef, of very common occurrence in the 

 Pacific and Indian Oceans, which goes by the name of 

 an " Atoll." This is, to all intents and purposes, an 

 encircling reef, without anything to encircle; or, in 



