XIV CORAL REEFS FOSSIL MADREPORARIA 393 



Indian Ocean. Here the action of the currents appears to have 

 cut down a great tract of land to form a plateau more than 100 

 fathoms in depth. The outer rim of this plateau may have 

 grown in height by the deposit of the skeletons of surface- 

 swimming animals, and the skeletons of deep-sea corals, until it 

 reached a level where reef-forming corals can thrive. A certain 

 number of channels would be retained and even deepened as the 

 rim grew up, and thus the coral would eventually reach the 

 surface not as a single large atoll, but as a series of coral islands. 

 When the coral reef has thus reached the surface and cannot 

 grow farther in height, it spreads radially like a fairy ring on 

 the talus formed by broken corals that have fallen down the 

 slopes. The central parts, no longer protected by living organ- 

 isms, are continually subject to the solvent action of the sea water 

 penetrating the porous substratum, and sink to form the lagoon. 



It is not only in the reefs of the Indian Ocean, however, but 

 in many of the archipelagoes of the Pacific Ocean, where there is 

 evidence of very extensive elevation of the land areas in the 

 neighbourhood of atolls and barrier reefs, that the subsidence- 

 theory does not satisfactorily account for all the observed facts. 

 It appears probable, therefore, that although a gradual subsidence 

 of the land may have been the primary cause of coral reef 

 formation in some areas, similar reefs may have been formed in 

 other areas by other natural methods. 



Fossil Corals. — A great number of the genera of corals found 

 in the newer Tertiary deposits, and a smaller number of those occur- 

 ring in the older Tertiary and Cretaceous strata clearly belong to 

 families now represented by recent corals. In the earlier strata, 

 however, fossils are found which cannot be placed in our system 

 with any degree of certainty. Attempts have been made from 

 time to time to arrange these corals in their proper positions by 

 the careful study and comparison of their skeletal features, but 

 the reasons given are not convincing. The genus Sijringopora, 

 and the families Favositidae, Heliolitidae, and Coceoseridae have 

 been noticed in the chapter on Alcyonaria (pp. 343-346). The 

 family Zaphrentidae will be noticed when dealing with the order 

 Zoanthidea. 



Among the families of fossil corals of uncertain position which 

 may still be included in the order Madreporaria, the more 

 important are : — 



