88 Mr. J. Bryce on the Lignites and Altered Dolomites 



the eastern side of the quarry. Along the plane of contact with 

 the dike the limestone is altered to the state of a granularsac- 

 charine marble, which on the application of a slight pressure 

 crumbles into a fine powder. This is succeeded by a hard 

 crystalline marble, the crystals appearing in distinct flakes. 

 Between this and the first change, which is one of simple in- 

 duration, there are many gradations. Similar effects are 

 common at the contact of limestone with plutonic rocks; in 

 some localities they 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 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 earth's crust accom- 

 panied the eruption of these plutonic rocks, gaseous exhala- 

 tions might find their way from beneath, and introduce car- 

 bonate of magnesia and other substances into rocks near the 

 surface. In confirmation of this view, Mr. Phillips has shown 

 in his Geology of Yorkshire, that "common limestone is do- 

 lomitized 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 mag- 

 nesia may be sublimed by the action of great heat. (Rep. 

 Brit. Assoc, for 1835, Trans. Sect. p. 51 ; Phillips's Geology, 

 vol. ii. p. 98.) 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, 

 1 submitted two specimens of the rock to Mr. John Mac- 

 adam, lecturer on chemistry in this city, for examination with 

 reference to the presence or absence of magnesia. The fol- 

 lowing is Mr. Macadam's report; the specimen referred to 

 as No. 1 is the saccharine marble from contact with the 

 dike ; No. 2 is the unaltered limestone — both were average 

 specimens. 



" I have carefully subjected to chemical analysis the spe- 

 cimen of limestone No. 1, with special reference to the pre- 

 sence or absence of magnesia; and I find from the indications 

 given, that carbonate of magnesia constitutes about 2| per 

 cent, of the whole mass. The mineral is not, therefore, a 

 double carbonate of lime and magnesia. Its other and prin- 



