Hardman — On the Carboniferous Dolomites of Ireland. 717 



' 



I. II. 



In undissolved t„ c^i ,^\r.^ 

 Residue. j ^^ Solution. 



III. 

 Total. 



Carbonate of lime, . . 



,, ,, magnesia, . 



Ferric oxide and alumina, 



Insoluble residue (siLica, 



&e.), 



Grains. 



38-50 



31-25 



0-63 



0-48 



Grains. 



40-10 



29-65 



0-33 



Grains. 



78-60 



60-90 



0-96 



0-48 



70-86 ! 70-08 



140-94 



TJie above reduced to Percentage Composition. 



Carbonate of lime, . . 

 ,, „ magnesia, - 

 Fenic oxide and alumina, 

 Insoluble residue, . . 



54-33 



44 10 



0-88 



0-67 



57-22 



42-31 



0-47 



55-76 



43-20 



0-68 



0-34 



99-98 



100-00 



99-98 



So far from the carbonate of magnesia being the most soluble here, 

 it will be seen that the result of the experiment has been actually to 

 bring the composition of the magnesian limestone nearer to that of 

 true dolomite than it was before. The proper proportions being 

 about 52-08, Ca CO3 to 46-50, Mg CO3. It will also be observed 

 from column II. that the carbonate of lime dissolved was much in 

 excess of carbonate of magnesia.] 



It had, some time before, struck me that if magnesian carbonate 

 were really more soluble under the circumstances which occur in 

 nature than carbonate of lime, we ought to find some account of it in 

 the stalactites and stalagmites so invariably found where water has 

 percolated through limestones. It has been long known that these 

 accumulations are usually free from magnesia, and the Messrs. 

 Eogers, in the paper already cited, refer to this as a proof of the 

 greater solubility of the carbonate of magnesia, since they say the 

 latter is carried off in solution, while the carbonate of lime is depo- 

 sited."^' jSTow it is difficult to imagine that all the carbonate of mag- 

 nesia would so completely disappear, and one would rather suppose 



I 



* Biscbof , in describing the mode of formation of the spnidelstone from the 

 Carlsbad hot springs, appears to coincide with this opinion, since he considers the 

 magnesia to be carried away wholly in solution. In this case, however, the water 

 has a ready means of escape, and the deposition of carbonate of Hme is due to 

 loss of carbonic acid, and not to evaporation. 



