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SECTION XIII. 



NOTE ON THE CORAL ROCK OF FUNAFUTI. 



By H. C. SoRBY, LL.D., F.R.S. 



The special points on wliich I propose to treat are the production of the aragonite 

 and of the dolomite in the coral rock. 



In the first place we must consider the formation of aragonite. Pseudomorphs of 

 calcite after aragonite are known, and it is easy to alter the latter into calcite by 

 artificially heating it. I have also altered the aragonite of pearl into well crystallised 

 calcite, which is a change so common in fossils. So far as I am aware, no pseudomorphs 

 of aragonite after calcite are known, nor any artificial ]neans of alterinii- it into aragonite. 

 Many years ago I made experiments in connection with the production of aragonite, 

 and though I found that, as is well known, it can be formed at a temperature 

 considerably above the natiual, I never adequately cleared up the question as to the 

 lowest limit at which it coukl be produced. I, however, did satisfy myself that 

 under special conditions it could be formed at the ordinary temperature. Thus, if a 

 portion of a crystal of gypsum is kept for a time in a fairly strong solution of 

 carbonate of soda, it is changed into a crystalline mass of carbonate of lime, which 

 I had reason to think was aragonite. I was led to this conclusion by finding that, 

 when taken out of the st)lution of carbonate of soda and put into water and watched 

 under a microscoj)e. the crystals were seen to gradually change into a mass of much 

 smaller crystals, which I took to be due to the aragonite changing into calcite. It 

 is, however, clear that ])otli these changes are abnormal, and depend on particular 

 conditions determined by the ])resence of carbonate of soda. The formation of 

 aragonite at the ordinary temperature also occurs in corals and in many shells, though 

 in particular layers of the self-same shells the carbonate of lime crystallises as calcite. 

 The point of interest of these facts, in connection with my present subject, is tliat 

 there may be special conditions not fully understood, under A\iiicli carbonate of lime 

 may crystallise as aragonite at such a temperature as Avould be met with in coral 

 rock. 



There is .dso another })oint that ought to be considered. Since aragonite is more 

 dense than calcite, it seems reasonable to suppose that its formation would be favoured 

 by pressure, so that it is possible that some unknown conditions, combined Avith 

 pressure, may have caused the dejjosition of aragonite in the cavities of the coral 

 rock, though possibly they could not cause the alteration of jireviously formed calcite 

 into arajTonite. 



