57 



found ; the unaltered rock is a dolomite, and contains nearly 34 per 

 cent, of carbonate of magnesia, while the altered rock contains less 

 than 3 per cent. What has become of the constituent magnesia ? 

 Has it been driven off by the heat to which the limestone was 

 exposed ? Most chemists are unwilling to admit that this is pos- 

 sible ; and it may reasonably be objected, that if the limestone had 

 been exposed to so high a temperature as to vaporize its magnesia, 

 the silica would not be mechanically present, but would have entered 

 into chemical combination with the lime or the magnesia, and have 

 formed a silicate. 



That whin dikes have sometimes been the means of producing 

 such a combination has been shown by an eminent chemist. In a 

 valuable paper by Dr. Apjohn on the Dolomites of Ireland, published 

 in the Dublin Geological Journal, vol. i., the details of an analysis 

 of the white chalk of Antrim, altered to the state of a saccharine 

 marble, are given (p. 376) ; and it is remarked in conclusion, that 

 " the stone under consideration consists of silica, combined with the 

 mixed oxides of calcium, magnesium, and iron (the carbonate of lime 

 being mechanically present), and is therefore a mixture of trisilicates, 

 very analogous in its composition to olivine. We are thus enabled 

 to understand why olivine should be so very frequently found in 

 trap rocks, and to refer its origin to the contact of silex at a high 

 temperature with an excess of the basic oxides ; and we have in some 

 degree a demonstration that the dolomites which contain siliceous 

 sand could not have been exposed at any time to a heat sufficiently 

 high to account for the introduction into them of magnesia in the 

 vaporous state ; for by such a heat a silicate of lime or magnesia, or 

 of both, would have been produced." 



The presence of these silicates in both our specimens is shown by 

 the appearance of the gelatinous silica ; yet a greater quantity of 

 silica is present mechanically, which, as already stated, seems incon- 

 sistent with the exposure of the rock to intense heat ; unless, indeed, 

 we could suppose that the silica has been introduced by infiltration, 

 or the magnesia removed by the solvent power of free carbonic acid 

 at a period subsequent to the consolidation of the dike from a state 

 of igneous fusion. 



Careful quantitative analyses of the limestones were afterwards 

 obtained through the kindness of Dr. Eobert D. Thomson. It ia 

 hoped that these will afford definite terms of comparison with other 

 analyses, such as those of Dr. Apjohn already referred to ; and that 

 their publication may lead to the formation of clearer views respect- 

 ing an obscure question in theoretical geology. 



