730 Proceedings of the Royal Irish Academy. 



so common to dolomites, and the filling up or coating of these cavities 

 \rith carbonate of lime. 



The frequent occurrence of quantities of carbonate of zinc in dolo- 

 mites may also be explained thus : the concentration, simultaneously, 

 of the carbonates of zinc and of magnesia being accomplished by the 

 removal of the carbonate of lime.* The resulting dolomite being 

 then less soluble that the carbonate of zinc, the latter would be dis- 

 solved out and again deposited alone in the lower cavities of the 

 rock. 



On the other hand, the percolation of water containing carbonate 

 of magnesia would add to the bulk of the mass, lyiless something was 

 abstracted in place of the carbonate of magnesia deposited. This 

 could only be carbonate of lime, but Bischof's experiment is against 

 that. Besides, as water usually contains about ten times as much 

 carbonate of lime as of magnesia, were any deposition to take place, 

 the lime would certainly be deposited before the mag-nesia, and would 

 not only increase the bulk, but neutralise the dolomitization. 



* I have already pointed ont ttat zinc is nearly always associated with, or an 

 accessory of, magnesian minerals. Vide On the supposed Substitution of Zinc for 

 Magnesium in Minerals, Proc. Eoy. Ir. Acad., 1874. 



