of the Island of Bute. 89 



cipal ingredients are carbonic acid and lime, besides which 

 silica is present, as also traces of oxide of iron and alumina. 



" In the specimen No. 2 I find magnesia in great abun- 

 dance, the amount present being equivalent to 33-72 per cent, 

 of carbonate of magnesia. The other constituents present are 

 similar to those reported in No. 1. From the large propor- 

 tions of carbonate of lime and carbonate of magnesia present 

 in specimen No. 2, it would appear to be a species of dolo- 

 mite. It may be noticed that the physical characters of No. 2 

 are very different from those of No. 1 ; the former is difficult 

 to pulverize, the latter is extremely susceptible of division. 



"The action of strong hydrochloric acid on both specimens 

 causes a portion of gelatinous silica to appear, showing the 

 presence of a silicate, which may be that of magnesia, since 

 the quantity of gelatinous silica is about sufficient to combine 

 with the r28 per cent, of caustic magnesia existing in the 

 specimen No. 1. There is a less quantity of this gelatinous 

 silica in No. 2. The greater portion, however, of the silica 

 present in both specimens remains undissolved in the gritty 

 or pulverulent condition, and is hence in a state of mere me- 

 chanical mixture with the other constituents 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 phaenomena are thus of a contrary character to what 

 I had anticipated; the unaltered rock is a dolomite, and con- 

 tains 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 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 pro- 

 ducing such a combination has been shown by an eminent 

 chemist. In a valuable paper by Dr. Apjohn on the Dolo- 

 mites of Ireland, published in the Dublin Geological Journal, 

 vol. i., the details of an analysis of the white chalk of An- 

 trim, 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 



