H4 ANCIENT AND MODERN CORAL GROWTHS. 



British limestone largely formed of corals is the Wenlock limestone ; 

 and here the formation is composed largely of starlike corals of the 

 genera Heliolites, Favosites, and Coanites, together with several which 

 form more or less expanded masses, such as Syringopora and Halysites, 

 Arachnophyllum and Syringophyllum. Again, in the middle of the 

 Devonian formation, the Plymouth limestone often consists to a large 

 extent of corals, among which are many genera, such as Favosites and 

 Heliolites, which first appeared in the Wenlock limestone, with the 

 addition of new forms, such as Smithia, &c. In the Carboniferous 

 Limestone, especially in parts of Derbyshire, corals have accumulated 

 so as to build up a considerable thickness of rock. Among these are 

 the genera Nematophyllum, Stylaxis, Strombodes, Likoshotia, Cyatho- 

 phyllum, and Michelenia. In the Secondary strata, especially of this 

 country, corals of a compound type are less numerous ; and although 

 they have been met with in isolated masses in the Lias, Inferior and 

 Great Oolite, it is only in the Coral Rag that limestone has been formed 

 by their growth and from material derived from corals worn up by the 

 sea. And similarly in the British Tertiary rocks there seems to have 

 been no period during which corals formed reefs, although in the 

 newer beds of the Lower Tertiary series, several examples of compound 

 corals are found. 



In existing seas, the most northern point at which reef-building 

 corals live is the Bermuda Islands, which lie in the direct course of 

 the Gulf Stream, and, therefore, in water warmer than is usual in 

 that latitude. But, speaking generally, reef-building corals range 

 between nearly 30 north of the equator and about 25 south of it. 

 They are considered by Professor Dana l to die when the temperature 

 of the sea falls below 66, and they are not found alive at a greater 

 depth than twenty-five fathoms. This depth they reach at the equator, 

 probably because the water there at that depth is warmed to the tem- 

 perature necessary to their existence. 



All the coral reefs of the world belong to one type, but they 

 have been classified by Mr. Darwin 2 for convenience in illustrating 

 questions of movements of the earth's crust into three groups, which 

 he names fringing reefs, barrier reefs, and atolls. 



The Fringing Eeef, under certain circumstances of depression of 

 land, may in time become converted first into a barrier reef, and at 

 last may take the form of an atoll ; so that no broad distinction can 

 be drawn between these forms of reefs, which succeed each other in 

 much such a way as childhood passes into youth, and youth into man- 

 hood. The fringing reefs are so named, because they grow as a 

 fringe around the shore, usually in shallow water where the sea- 

 bed slopes at a slight angle. They grow at varying distances from 

 the coast, which depends upon depth and the amount of sediment 

 derived by tidal waters from the shore. The reefs are usually laid 

 more or less bare at low water, when the bright-coloured red or green 

 living animals are seen to rise just out of the water like a mud bank, 

 through which numerous channels of water extend to the ocean. 

 1 Coral Islands. * Coral Reefs. 



