208 Mb. Brtce on the Geology of t/ie Island of Bute. 



and is hence in a state of mere mechanical mixture with the other con- 

 stituents of the limestone. It would require a minute quantitative 

 analysis to determine whether the 1'28 per cent, of magnesia exists as a 

 carbonate or silicate, or partly as both." 



The phenomena are thus of a contrary character to what I had 

 anticipated, — the unaltered rock is a dolomite, and contains nearly 

 34 per cent, of carbonate of magnesia, while the altered rock con- 

 tains less than 3 per cent. What has become of the constituent mag- 

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

 exposed ? Most chemists are unwilling to admit that this is possible; 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. 1st, 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 i3 

 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 suffi- 

 ciently 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 

 gelatinous silica appearing; yet a greater quantity of silica is present 

 mechanically; which, as already stated, seems inconsistent 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. It is unne- 

 cessary, however, to pursue the subject farther with our present limited 

 knowledge of facts ; it is one of great interest both to the chemist and 

 the geologist, and as no instance of similar changes on dolomitic rocks has, 

 so far as I am aware, ever been put on record, the subject is deserving 

 of a full investigation. I hope to be able, in the course of next 

 session, to lay before the society complete quantitative analyses of a 

 suit of specimens illustrative of the structure of the limestones of 

 Bute, and the nature of the metamorphic action to which they have been 

 subjected. 



