1 66 MARINE DEPOSITS 



to them all. In this compound mass is secreted a skeleton of 

 carbonate of lime, which reproduces the form of the colony and, 

 in most cases, displays cells for the individual polyps. The great 

 variety of form shown by these compound colonies is determined 

 by the mode of budding or fission and the relative position of the 

 newer to the older polyps. Thus, some are like trees, others like 

 bushes ; some form flat, irregular plates, while others grow into 

 great dome-like masses. 



The reef corals have, at present, a restricted distribution, and 

 can flourish only where several favourable conditions are found 

 united. They are preeminently shallow- water animals and can live 

 only in depths of less than twenty fathoms. They also require a 

 high temperature, and they cease wherever the average tempera- 

 ture of the water for the coldest month is below 68° F. ; this is the 

 minimum, and for full luxuriance a higher temperature is neces- 

 sary. Another requisite is sea-water of full salinity and uncontami- 

 nated with mud ; hence, corals cannot live at the mouth of a river, 

 which, even if it brings down no sediment, freshens the water and 

 is thus fatal to the polyps. Another condition favourable to the 

 growth of corals is the presence of ocean currents, not too rapid, 

 which bring abundant supplies of food, and they flourish best in 

 the broken waters of heavy surf, which gives the necessary oxygen 

 and prevents the smothering of the polyps in the calcareous silt 

 and debris of the reef. In short, the reef corals are tropical, 

 marine, shallow-water animals, and their reefs are widely spread 

 throughout the warmer seas of the globe, but they do not always 

 occur where we should naturally expect to find them. 



A coral reef is not built, as many people imagine, by the indus- 

 try of the polyps — these furnish the material, by extracting lime 

 salts from the water and forming solid skeletons ; the actual 

 construction is largely the work of the waves, for the corals live 

 within the limits of wave action. The coral colonies are scattered 

 over the sea-bottom, much like vegetation on the land, scantily in 

 some places, thickly in others, and in still others they are absent. 

 The waves, especially in storms, break up the masses of coral, 

 which are much weakened by the borings of many kinds of marine 



