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Hardman — On the Carboniferous Dolomites of Ireland. 723 



in few instances any information as to the composition of the rocks 

 over or thi'ongh which they passed ; and these rocks may have con- 

 tained but a trace of magnesia. But we can assert this much ; that 

 ■on the generally received notion as to the relative solvency of those 

 bodies, and with which, save in the event of their being combined in 

 rocks — when it certainly seems to reverse its behaviour — I must coin- 

 cide, the result of such waters percolating through rocks would be, if 

 anything, to form limestone, and not dolomite, since the carbonate of 

 lime being more abundant, as well as less soluble than the carbonate 

 of magnesia, would be more likely to be deposited. In all these sup- 

 posed infiltration theories too, the bulk of the rock would necessarily 

 be increased, unless it is taken for granted that some of the carbonate 

 ■of lime is removed in the process, and replaced by carbonate of mag- 

 nesia. 



The only way in which the production of at least the Irish dolo- 

 mites can be accounted for is, by the gradual removal of the excess of 

 carbonate of lime. It is quite possible, and indeed likely, that in 

 such a process, as the solution contains some carbonate of magnesia, 

 a part of it in the form of dolomite may be deposited in a different 

 part of the rock from which it was derived ; the waste of one portion 

 going to help to build up another."^'' 



One point in favour of the abstraction theory is, that the Irish dolo- 

 mites are exceedingly porous, cellular, or cavernous. Another curious 

 point is that the cavities are almost invariably filled with calcspar ; 

 and not bitter spar or dolomite, as is generally stated. I have carefully 

 examined the dolomitic limestones which occur so plentifully in the 

 Counties Carlow and Kilkenny, and are spread over a large area, as 

 well as some in the Counties Waterford, and Tyrone ; and I can safely 

 say that in no case have I found specimens of dolomite or magnesite 

 in the cavities, but, on the contrary, calcspar most abundantly. 



At Drumreagh, near Coal Island, Co. Tyrone, several beds of dolo- 

 mite are interbedded with the ordinary blue fossiliferous limestone, 

 and one of the beds merges gradually into the limestone, showing con- 

 clusively that it is metamorphosed limestone, and not the result of 

 original chemical deposition on the sea bottom, according to Dr. Sterry 

 Hunt's theory of such rocks. It is a light brown, extremely hard, 

 crystalline, but compact dolomite ; but so very cavernous that it is most 

 difficult to obtain a fracture of it showing the true structure. The 

 cavities are often large, as much as a foot in diameter, and coated, or 

 often entirely filled, with nearly pure calcite, which may be obtained 

 in large, nearly transparent rhombohedrons. The dolomite is per- 

 fectly unaffected by acid, in the cold. I should say that fully half 

 the original rock is wanting, being now only represented by the spar- 

 coated cavities. 



I think Bischof makes a similar suggestion. 



