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CHAPTER III. 

 FOSSIL CORALS. 



PERHAPS no fossils have such a geological value as 

 corals. If extinct species were marked by the same 

 habits as their modern representatives (and in many 

 cases the families of living corals are so ancient, and 

 the extinct forms glide so imperceptibly into existing 

 kinds, that there is no absolutely strongly marked line 

 of division), then their value to the physical geologist 

 who endeavours to restore the conditions of primeval 

 seas is immense. For coral-animals can only flourish 

 where the sea-water is clear, and therefore where no 

 muddy sediments are forming. Coral-animals are 

 easily separable into two groups the single or 

 simply compound corals, which are usually inhabitants 

 of deeper water ; and the reef-building corals, which 

 cannot live and flourish beyond the depth of twenty- 

 five fathoms. Moreover, according to Darwin, coral 

 reefs indicate to the physical geographer slowly sub- 

 siding areas of the sea-floor. They are also indicative 

 of a certain degree of ocean temperature, for we do 



