328 GEOLOGY OF BUTE. 



chemist. Dr. Apjohn found the white chalk of Antrim, 

 altered by whin dikes to the state of a saccharine marble, to 

 be a trisilicate of lime, " very analogous in its composition 

 to olivine. We are thus enabled to understand why olivine 

 should be so very frequently found in trap rocks, and to 

 refer its origin to the contact of silex at a high temperature 

 with an excess of the basic oxides; and we have in some 

 degree a demonstration that the dolomites which contain 

 siliceous sand could not have been exposed at any time to a 

 heat sufficiently high to account for the introduction into 

 them of magnesia in the vaporous state; for by such a heat a 

 silicate of lime or magnesia, or of both, would have been pro- 

 duced " (Jour. Geol. Soc. Dub., vol. i. p. 376). 



The presence of these silicates in both our specimens is 

 shewn by the appearance of the gelatinous silica; yet a 

 greater quantity of silica is present mechanically, which, as 

 already stated, seems inconsistent with the exposure of the 

 rock to intense heat; unless, indeed, we could suppose that 

 the silica has been introduced by infiltration, or the magnesia 

 removed by the solvent power of free carbonic acid at a period 

 subsequent to the consolidation of the dike from a state of 

 igneous fusion. 



139. Careful quantitative analyses of the limestones were 

 made by the late Dr. Robert D. Thomson. It is hoped that 

 these will afford definite terms of comparison with other 

 analyses, such as those of Dr. Apjohn, already referred to, 

 and that their publication may lead to the formation of 

 clearer views respecting an obscure question in theoretical 

 geology. 



The analyses are as follow : 



Specimen No. 1 is the saccharine marble from contact with 

 the dike at Kilchattan in the highest state of alteration. 



No. 2 is the hard crystalline marble, having the crystals in 

 distinct flakes, more remote and less altered than No. 1. 



No. 3 is the unaltered limestone from the middle of the 

 quarry, remote from the dike an average specimen. 



