Hardman — On the Carbon if erovs Dolomites of Ireland. 72^ 



Crystallization. — The very distinct crystalline appearance of dolo- 

 mite is a matter requiring important consideration, but it is in truth 

 one of the chief diiliculties of the whole question. From whatever 

 stand-point we approach the subject, and whatever the theory which 

 we adopt may be, this is not very easy to account for. The infiltra- 

 tion of carbonate of magnesia could hardly of itself afford this peculiar 

 structure, for it would only give a perimorph, or at least a^ pseudo- 

 morph, of magnesite, by replacing part of the carbonate of lime in the 

 calcite. But even magnesite after calcite is not frequent. Blum and 

 others refer to it as occasionally occurring in lodes, and in geodic cavities; 

 but it is not likely often to be discovered, for Bischof found it impos- 

 sible to effect any decomposition between carbonate of lime, and a 

 solution of bicarbonate of magnesia.* 



On the other hand, all limestones, with the exception of the earthy 

 varieties, are more or less crystalline, and the crystals of calcite differ 

 only to a very slight amount from those of dolomite — so little that the 

 principal angle of the rhombohedron of calcite being 105°5', that of 

 dolomite is 10 6° 15', a difference quite inappreciable, without the aid 

 of delicate instruments. This being so, if a quantity of superabun- 

 dant carbonate of lime be removed from a highly magnesian limestone, 

 such as would, according to Bischof, be formed by the agency of certain 

 organisms, the crystalline structure would appear very distinctly^ 

 even in magnesian limestones that were still far removed from dolo- 

 mites. In fact in few dolomites are the crystals really distinct until 

 the rock has begun to decompose, and I could point out many loca- 

 lities near Kilkenny where true dolomites are perfectly compact, to all 

 appearance, where unweathered, but once attacked by the atmosphere 

 show themselves to be highly crystalline ; the process being just 

 what I have suggested above, viz., the removal of the superfluous 

 carbonate of lime. This is, however, an extremely difficult province 

 of the question to enter upon, and an opinion on it is not to be 

 advanced without great diffidence, during the existing state of our 

 information about it. 



Conclusions. — (1). It appears, therefore, that the Irish carbonife- 

 rous dolomites could not have been completely originated by organic 

 agency, nor could they have been formed by chemical deposition 

 due to evaporation of sea water ; and there seems to be evidence of 

 few other dolomites being formed in the latter way. 



(2). The experiments recorded in the preceding paper, showing the 

 much greater solubility of carbonate of lime than of carbonate of mag- 

 nesia, fi'om rocks treated with a carbonic acid solution, appear to bear 

 out the theory that dolomite may be formed by the extraction — by 

 water holding in solution small quantities of carbonic acid — of the 

 excess of carbonate of lime from magnesian limestone rocks. 



In this way, also, it is easy to account for the fissures and cavities 



* See Xote to p. 708. 



