SOME METASOMATIC CHANGES IN LIMESTONES. 55 



Devonian and Carboniferous of the Ardenne, has specially 

 remarked the dolomitisation of the central parts of the reefs. 

 In the emerged coral-island of Metia or Matea, in the 

 Pacific, Dana has pointed out that the interior, i.e., the part 

 containing the lagoon, is of dolomite-rock with up to 38 per 

 cent, of magnesium carbonate, while the corals of the same 

 reef contain only a trace of magnesia, and he inferred the 

 introduction of magnesia into the rock trom sea-water, 

 probably concentrated in a closed lagoon, the process taking 

 place therefore at ordinary atmospheric temperatures. These 

 various facts point to some peculiarity in certain limestones 

 rendering them susceptible to dolomitisation, a peculiarity well 

 exhibited in — though not confined to — rocks of coral origin. 



Now the important chemical difference between corals 

 and many other calcareous organisms is that the calcium 

 carbonate in the former is not calcite but aragonite, and 

 Klement was accordingly led to experiment on the action of 

 sea-water on aragonite. The results of his experiments 

 may be summarised as follows : — 



Powdered aragonite was acted upon by magnesium 

 sulphate in a saturated solution of sea-salt, the action being 

 found to take place at temperatures above 6o° C. The 

 amount of magnesium carbonate formed was found to in- 

 crease with the temperature, but there appeared to be a 

 certain maximum for each temperature. For gi u the 

 maximum was about 42 per cent, of magnesium carbonate, 

 which is not much less than that in pure dolomite (457). 

 In dilute solutions of sea-salt the action was found to be 

 much feebler, and it was also feebler when too much of the 

 solution was used, apart from its degree of concentration. 

 In the absence of sodium chloride the action of the mag- 

 nesium sulphate, even in concentrated solution, was feeble. 

 Magnesium chloride, substituted for the sulphate, was found 

 to be much less active. The sulphate had very little action 

 on calcite, but acted on corals just as on artificial aragonite. 



These experiments go to establish that the magnesium 

 sulphate in sea-water will act upon corals or other aragonite 

 organisms in a warm climate, and in places where the salt 

 in the sea can reach a considerable degree of concentration. 



