GEOLOGY OF BUTE. 327 



" In specimen No. 2, magnesia abounds, the amount pre- 

 sent being equivalent to 33 '72 per cent, of carbonate of 

 magnesia. The other constituents are similar to those in. 

 No. 1. From the large proportions of carbonate of lime and 

 carbonate of magnesia, No. 2 would appear to be a species 

 of dolomite. 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, shewing the 

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

 the quantity of gelatinous silica is about sufficient to combine 

 with the 1'28 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 

 mechanical mixture with the other constituents of the lime- 

 stone. 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 

 is usually found; 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 vapourize its magnesia, 

 the silica would not be mechanically present, but would have 

 entered into chemical combination with the lime or magnesia, 

 and have formed a silicate. 



That whin dikes have sometimes been the means of pro- 

 ducing such a combination has been shewn by an eminent 



