GEOLOGY. 277 



a similar character, it is necessary that there be animal exuvia in coral 

 mud or finely divided carbonate of lime. Both these occur. The 

 water about the keys abounds in animal life. 



With the influx of the tide the slopes become overspread with the 

 water and what it contains in suspension. The retreating water, at 

 ebb tide, leaves a thin layer of the animal matter, mixed always, when 

 the water is agitated, with the fine calcareous powder. Before the 

 return of flood tide, exposure to the atmosphere and warmth have 

 secured the succession of chemical changes enumerated above, and a 

 thin layer of rock is formed. A repetition of this process makes up the 

 numerous excessively thin layers of which this rock is composed. On 

 the ocean side, the deposit is formed from spray during winds which 

 drive the froth of the sea, containing, with coral mud, the exuvia from 

 the barrier of living corals upon the low blufls of the keys. 



To these chemical changes must be added the simple admixture of 

 the animal and vegetable matter, which, like mucilage or glue, fills up 

 the interstices, increases the extent of surface, and with it the cohesive 

 attraction, and still further to the decomposition of the organic matter 

 furnishing carbonic acid, which gives solidity to the pulverulent car- 

 bonate of lime. The exceeding fineness of the coral mud is due in part 

 to the stone plants which flourish in the waters within the reef, and 

 which admit of ready reduction to a powder of extreme fineness. 



No alumina was found in any of the corals analyzed. The stone 

 plant as well as the coral animal possesses the power of abstracting 

 lime from sulphuric acid the change doubtless being due to double 

 decomposition with carbonate of ammonia excreted from the plant and 

 animal, yielding carbonate of lime quite insoluble and sulphate 

 of ammonia of the highest solubility. The building up of the calca- 

 reous skeleton becomes, therefore, of exceeding simplicity. The sur- 

 rounding element yields at once to the exhaling carbonate of ammonia 

 the framework of stone. 



The conclusions to which the above research has conducted, are : 



That the submerged or oolitic rock has been solidified by the infil- 

 tration of finely powdered (not dissolved) carbonate of lime, increas- 

 ing the points of contact, and the introduction of a small quantity of 

 animal mucilaginous matter, serving the same purpose as the carbonate 

 of lime that of increasing the cohesive attraction ; and 



That the surface rock has been solidified by having, in addition to 

 the above agencies, the aid of a series of chemical decompositions and 

 recompositions resulting in the formation of a cement. 



It is one of the grateful reflections arising from this inquiry, that 

 nature has provided, in the very conditions of the growth of the coral, 

 the means of securing, and, in the progress of ages, rendering habit- 

 able, the domain cut off from the sea by the emergence of coral reefs. 



THE COMPLETED CORAL ISLAND. 



THE coral island, in its best condition, is but a miserable residence 

 for man. There is poetry in every feature ; but the natives find this a 

 poor substitute for the bread-fruit and yams of more favored lands. 



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