326 GEOLOGY OF BUTE. 



are accompanied by other singular changes of a chemical 

 nature. Magnesia, and sometimes silica and alumina, are 

 introduced into the composition of the limestone, so that 

 simple carbonate of lime becomes a double carbonate of lime 

 and magnesia. The source 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 earth's crust accompanied 

 the 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 confirmation of this view, Mr. Phillips has shewn, in his 

 Geology of Yorkshire, that " common limestone is dolomitized 

 by the sides of faults and mineral veins far away from igneous 

 rocks of any kind ;" and some distinguished chemists have 

 expressed their belief that carbonate of magnesia may be 

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

 1835, Trans. Sect, p. 51 ; Philips's Geology, in Cab. Cyclop., 

 vol. ii. p. 98.) Much doubt, however, still hangs about 

 this subject. Cases occur in which magnesia has been in- 

 troduced, 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. 



In order to elucidate, if possible, this obscure subject, two 

 specimens of the rock were submitted to the late Professor 

 Macadam, at that time lecturer on chemistry in Edinburgh, 

 afterwards Professor in Melbourne College, for examination 

 with reference to the presence or absence of magnesia. 

 Specimen No. 1 is the saccharine marble from contact with 

 the dike; No 2 is the unaltered limestone both average 

 specimens : 



" In specimen No. 1, carbonate of magnesia constitutes 

 about 2 per cent, of the whole mass. Its other and 

 principal ingredients are carbonic acid and lime, silica, and 

 traces of oxide of iron and alumina. 



