258 CCELENTERATA. 



be more or less obliterated by secondary crystallisation. In some cases, 

 where the rock is composed of large fragments of coral cemented to- 

 gether by calcareous debris, a "coral-breccia" is produced. A some- 

 what similar rock (" boulder-rock ") is formed above the level of high 

 water by the fragments of coral which are heaped up by the waves, and 

 which become ultimately cemented together by the action of the spray. 



3. " Coral-mud" formed of fine calcareous mud derived from the wear 

 and tear of the reef by the action of the waves. Such deposits are neces- 

 sarily stratified, and they are more or less fine-grained, chalk-like in 

 appearance, and often friable. Though largely composed of minute 

 particles of carbonate of lime, these coral-muds are often extensively 

 made up of recognisable fragments of various calcareous organisms, or 

 of the minute tests of Foraminifera. 



4. " Sand-rock" formed above high-water mark by the action of the 

 wind. Though subaerial in origin, the " sand-rock " is stratified ; and it 

 is composed of fine coral sand cemented together by the action of the 

 spray of the sea, or by the percolation through it of rain-water. 



Coralline limestones have been formed in all the great geological 

 periods from the Ordovician onwards, and some of these undoubtedly 

 represent ancient coral-reefs. In an old coral-reef we should expect 

 to find the same two groups of calcareous rocks as have been above 

 noted as occurring in modern reefs. We ought,' namely, to meet in 

 a fossil reef, on the one hand, with limestones formed mainly of 

 corals which actually grew in place, and, on the other hand, with 

 limestones produced by the contemporaneous degradation of the 

 reef and therefore made up of the debris of corals and other cal- 

 careous organisms. Owing, however, to the action of denudation, 

 it must be a matter of comparatively rare occurrence that both these 

 groups of deposits should be found occupying their natural relative 

 positions at the existing surface ; and the rarity of such an occur- 

 rence must be in direct proportion to the antiquity of the supposed 

 reef. In any case, it can only be under exceptional circumstances 

 that the original form of any ancient coral-reef can have been so far 

 conserved that it would be possible to determine that the reef was 

 originally a "fringing reef," a "barrier," or an "atoll." In the 

 later Tertiary rocks, no doubt, denudation may have been com- 

 paratively speaking so slight that the original form of the reef may 

 admit of determination. Even in the Secondary rocks, however, 

 the changes produced by displacements of the strata and subse- 

 quent denudation are so great that the original form of the reefs 

 can hardly be more than a matter of inference. In the still more 

 ancient Palaeozoic deposits, again, the dislocations due to earth- 

 movements have been so extensive, and the denudation subsequent 

 to these has been so long continued and so effective, that it must 

 always be hazardous to treat the existing outcrops of coralline lime- 

 stones as representing the original boundaries of old coral-reefs. Hence 

 there is a considerable element of uncertainty attaching to the so- 



