708 Proceedings of the Royal Irish Academy. 



the action of Tvater holding carbonate of magnesia in solution, pene- 

 trating the rock, and depositing carbonate of magnesia, Tvhile at the- 

 same time removing a portion of the carbonate of lime ; or, as he- 

 admits, it may occiu' in some cases by the simple removal of carbonate- 

 of lime from a magnesian limestone by ^vater containing carbonic 

 acid ; the result being, of course, a gradual increase in the proportion 

 of carbonate of magnesia. While admitting this process, which Avas 

 first suggested by Grand] can, to be possible — as he shows by two 

 experiments, which prove carbonate of lime to be actually more- 

 soluble than carbonate of magnesia in water containing a small per- 

 centage of carbonic acid — he, however, appears to give the preference 

 to the first process, viz., the infiltration of carbonate of magnesia, 

 and removal of lime.^ But there appear to be one or two weak 

 points about this. 1st. That with so little diiference in solubility of 

 the two salts, a substitution of one for the other would hardly take 

 place to the extent required. f 2nd. That the lime removed must 

 always be equal — proportionally to theii' respective atomic weights — 

 to the magnesia deposited, or the rock would increase in bulk. 3rd. 

 That the result would only be a carbonate of lime with carbonate of 

 magnesia deposited in crevices or interspaces left by the removal of 

 the excess of carbonate of lime ; and 4th, that there is a difficulty 

 sometimes in imagining a sufficient supply of magnesianized water in 

 localities where, as in the central plain of Ireland, there are none but 

 limestone rocks, the water from which, containing a much larger quan- 

 tity of lime than magnesia, could hardly, therefore, produce the sup- 

 posed effect ; yet all these limestones are highly dolomitic. 



Any alteration that has taken place in these must have been entirely 

 produced by surface-water, or rain-water, which could contain little oi" 

 no constituents capable of affecting the limestone rocks, except car- 

 bonic acid. To the action of this agent I attribute the alteration 

 which most of the Irish limestones have undergone in theii- passage into 

 dolomite. At the same time, I think it quite possible that water 

 Mghly charged tcith carlonate of magnesia, which may be the case if it 

 has percolated a magnesian roclc, may deposit the magnesia while it 

 removes the lime, and thus aid in the metamorphism ; but, as I shall 

 show hereafter, waters containing any appreciable amount of car- 

 bonate of magnesia are rare; and after all, as Bischof shows, in such a 

 case the chief work is done by the action of the carbonic acid.t 



If we suppose a limestone rock, containing, as very many lime- 

 stones do, carbonate of magnesia to the extent of 12 per cent., to- 



* Chem. GeoL, vol. iii., p. 164. 



t Bischof failed, after "taking much pains," to effect the mutual decomposition 

 of carbonate of lime and carbonate of magnesia. In one case he digested fragments- 

 of chalk -^-ith pure carbonate of magnesia, for several years, without any effect. — 

 Op. cit., vol. iii., 167. 



X Chem. GeoL, vol. iii., p. 174. (In effect, although not in these precise tenns}^ 



