722 Proceedings of the Royal Irish Academy. 



that carbonated water may dissolve out only carbonate of lime until, 

 at last, the proper dolomitic proportions are reached. "* 



It will be noticed, however, that in the analyses given in the paper 

 just referred to, there is after all, in most of them, a considerable 

 range of proportion outside the dolomitic limit, and the mean given of 

 all the analyses shows that, in most cases, the magnesian carbonate 

 cannot have been present to an extent of more than 14 or 15 per cent., 

 and that in all, it is much less than the lime carbonate — thus com- 

 pletely verifying my experiments, and showing that those of Professor 

 Rogers are not based on natural processes. f 



I think, on the whole, it might be safely asserted that in every 

 case where atmospheric water acts on a limestone rock, it will remove 

 proportionately more carbonate of lime than carbonate of magnesia. 

 The reason for this it would be difficult to give, seeing that there can 

 be no doubt as to the somewhat greater solubility of magnesian salts 

 under laboratory conditions. 



If we suppose, however, that the carbonate of magnesia, in what- 

 ever proportion it is present in the rock, is originally combined as 

 dolomite, it might account for what otherwise appears to be an ano- 

 maly. Is there any difficulty in supposing that the small amount of 

 magnesian carbonate which it is known many corals and molluscan 

 shells contain, sometimes reaching as much as 7 '6 per cent, may 

 have been secreted as dolomite ? 



Forchammer has shown that some corals, annelids, and molluscan 

 shells, contain an appreciable quantity of carbonate of magnesia; in the 

 annelidse especially it being very high (7 '6 per cent.) Bischof, com- 

 menting on this fact, remarks, the limestones formed by serjnda;, 

 coralliiDn, isis, and probably other genera, ought to be termed dolo- 

 mitic limestone. J 



It is possible that many other organisms, such as build up rocks,, 

 secrete carbonate of magnesia to a perceptible amount. Many plants 

 also secrete carbonate of magnesia, and it is just possible that in such 

 cases the carbonate of lime and of magnesia may be combined as dolo- 

 mite. In such an event the removal of the excess of carbonate of lime, 

 which might in these instances be regarded as a matrix, would soon 

 result in a dolomite. 



It cannot be said that the foregoing analyses of waters prove much 

 with regard to the relative solubility of the carbonates, since we have 



* See Bischof, op. cit. iii., pp. 162, ]96, 200, 203, kc. Eischof does not,liOAr- 

 over, entirely favoiir tHs tlieoiy, which, is Grandjean's, not his own, but allows 

 merely that such a process is possible in some cases. 



t Bischof asserts that even springs rising in dolomite must always contain 

 more carbonate of lime than of magnesia, as, from his experiments, carbonated 

 water extracts little or no carbonate of magnesia from dolomitic rocks — op. cit., vol. 

 i.,p. 81. __ 



X Bischof, op. cit., To\ i.. p. 1S3 ; vol. u., pp. 48, 49. 



