404 CORAL AND CORAL REEFS 



there is comparatively little living coral ; the bottom of it 

 is formed of coral mud. If we pounded this coral in water, 

 it would be converted into calcareous mud, and the waves 

 during storms do for the coral skeletons exactly what we 

 might do for this coral in a mortar ; the waves tear off 

 great fragments and crush them with prodigious force, until 

 they are ground into the merest powder, and that powder 

 is washed into the interior of the lagoon, and forms a muddy 

 coating at the bottom. Beside that, there are a great 

 many animals that prey upon the coral fishes, worms, and 

 creatures of that kind, and all these, by their digestive 

 processes, reduce the coral to the same state, and contribute 

 a very important element to this fine mud. The living 

 coral found in the lagoon, is not the reef building coral ; 

 it does not give rise to the same massive skeletons. As 

 you go in a boat over these shallow pools, you see these 

 beautiful things, coloured red, blue, green, and all colours, 

 building their houses ; but these are mere tenements, and 

 not to be compared in magnitude and importance to the 

 masses which are built by the reef-builders themselves. 

 Now, such a structure as this is what is termed a " fringing 

 reef." You meet with fringing reefs of this kind not only 

 in the Mauritius, but in a number of other parts of the 

 world. If these were the only reefs to be seen anywhere, 

 the problem of the formation of coral reefs would never 

 have been a difficult one. Nothing can be easier than to 

 understand how there must have been a time when the 

 coral polypes came and settled on the shores of this island, 

 everywhere within the 20 to 25 fathom line, and how, 

 having perched there, they gradually grew until they built 

 up the reef. 



But these are by no means the only sort of coral reefs in 

 the world ; on the contrary, there are very large areas, not 

 only of the Indian ocean, but of the Pacific, in which many 

 many thousands of square miles are covered either with a 

 peculiar kind of reef, which is called the " encircling reef/' or 

 by a still more curious reef which goes by the name of the 

 4< atoll. There is a very good picture, which Professor 

 Roscoe has been kind enough to prepare for me, of one of 

 these atolls, which will enable you to form a notion of it as a 

 landscape. You have in the foreground the waters of the 

 Pacific. You must fancy yourself in the middle of the great 

 ocean, and you will perceive that there is an almost circular 

 island, with a low beach, which is formed entirely of coral 



