Mr. Bryce on the Geology of the Island of Bute. 207 



tho limestone, so that simple carbonate of lime becomes a double carbo- 

 nate of lime and magnesia. Tho question whence this magnesia has been 

 derived, has occasioned much difference of opinion among geologists. 

 Some imagine that it has been transferred from the plutonic rock to the 

 limestone ; while others hold that, as fractures and dislocations of the 

 {Mirth '• crust accompanied tho eruption of these plutonic rocks, gaseous 

 exhalations might find their way from beneath,, and introduce carbonate 

 of magnesia and other substances into rocks near the surface. In con- 

 firmation of this view, Mr. Phillips has shown, in his Geology of York- 

 shire, that " common limestone is dolomitized by the sides of faults and 

 mineral veins far away from igneous rocks of any kind ;" and some distin- 

 guished chemists have expressed their belief that carbonate of magnesia 

 may bo sublimed by the action of great heat. (Rep. Brit. Assoc, for 

 1835, trans, sect. p. 51 ; Phillips's Geology, vol. IL p. 9&) Much 

 doubt, however, still hangs about this subject. Cases occur in which 

 magnesia has been introduced, although the limestone could not have 

 been subject to such a pressure as would confine its carbonic acid when 

 the rock was softened by heat. 



Being anxious to elucidate, if possible, this obscure subject, I submitted 

 two specimens of the rock to Mr. John Macadam, lecturer on chemistry, 

 60 High John-Street, for examination with reference to the presence or 

 absence of magnesia. The following is Mr. Macadam's report; tho speci- 

 men referred to as No. 1 is the saccharine marble from contact with the 

 dike; No. 2 is the unaltered limestone from the western part of the 

 quarry ; both were average specimens. 



" I have carefully subjected to chemical analysis the specimen of lime- 

 stone No. 1, with special reference to the presence or absence of magnesia ; 

 and I find from the indications given, that carbonate of magnesia consti- 

 tutes about 2J per cent, of the whole mass. The mineral is not, therefore, 

 a double carbonate of lime and magnesia. Its other and principal 

 ingredients are carbonic acid and lime, besides which silica is present, as 

 also, traces of oxido of iron, and alumina. 



" In the specimen No. 2, I find magnesia in great abundance ; the 

 amount present being equivalent to 33*72 per cent, of carbonate of mag- 

 nesia. The other constituents present are similar to those reported in 

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

 magnesia present in specimen No. 2, it 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 pulverise, 

 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 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 

 apeoimenfl remain undissolved, in the gritty or pulverulent condition; 



Vol. II.— No. 1. 2 



