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THE POPULAR SCIENCE MONTHLY. 



coral where the work of growth goes on. The inner or dead portions 

 constitute the stony mass on which new material is secreted, but is no 

 more essential for that purjiose than a rock or a sandy shore. Prof. 

 Dana observes that, if the living portion could be separated, it would 

 form a hemispherical shell about half an inch thick. As the higher 

 orders of trees increase in size by additions of new wood at the outer 

 margin of the trunk, long after the beart-wood is dead, so the coral 

 is alive and grows only on its surface. 



It is obvious that the increase of the coral will continue without 



Fig. 11. 



Alcyonokl Polypes; "gayest and most delicate of coral shrubs." 



limit except from surrounding conditions. Thus, if the dome reaches 

 the surface of the water, the polypes die, and growth ceases in that 

 direction ; but it may increase in diameter, forming some remarkable 

 structures, which we will presently notice. But, if the coralline mass 

 be continually sinking by a subsidence of the land on which it rests, 

 the conditions of growth continue, and reefs of tremendous mass 

 and thickness are formed. 



The dead coral is always more or less porous, until the pores and 

 polype-cells are filled by comminuted substance or the infiltration of 

 carbonate of lime. Aided by chemical changes, the mass becomes solid 

 coral-rock, and finally compact limestone, with few traces remaining of 



