FOEMATION OP COEAL EEEFS. 



[Ch. XLIX. 



bottom 



and this IS hj no means a rare case. It follows that the reef 



Mr 



show that he understood it so. Noa\ 



exist under the sea, they ought somewhere to" b^e^ounron 



masses 



fi 



ubmer 



But neither 



m the great volcanic chain, extending from Sumatra to Japan 

 nor m the West Indies, nor in any other region yet explored' 

 has a bed or formation of coral even 500 feet thick been dis- 

 covered, so far as we know.' 



When considering the objection, it is evident that the 

 first question we have to deal with is, whether geologists have 



already discovered calcareous 



masses of the required 



thickness and structure, or precisely such as the upheaval of 

 atolls might be expected to expose to view ? We are called 

 upon, in short, to make up our minds both as to the internal 

 composition of the rocks that must result from the growth of 

 corals, whether in lagoon islands or barrier reefs, and the 

 external shape which the reefs would retain when upraised 

 gradually to a vast height, 



some may imagine. If the reader has pictured to him'self 

 large masses of entire corals, piled one upon the other, for a 

 thickness of several thousand feet, he unquestionably mistakes 

 altogether the nature of the accumulations now in progress. 

 In the first place, the strata at present forming very exten- 



means 



bottom 



New 



chiefly of horizontal layers of calcareous sediment, while here 



ill m . 



and there 

 oranitic 



intermixture must occur of the detritus of 

 other rocks brought down by rivers from the 

 adjoining lands, or washed from sea-cliffs by the waves and 

 currents. Secondly, in regard to atolls, the stone-making 

 polypifers grow most luxuriantly on the outer edge of the 

 island, to a thickness of a few feet only. Beyond this margin 

 broken pieces of coral and calcareous sand are strewed by the 

 breakers over a steep seaward slope, and as the subsidence 

 continues the next coating of live coral does not grow ver- 

 tically over the first layer, but on a narrow annular space 





i 



