90 Mr. J, Bryce on the Lignites and Altered Dolomites 



of trisilicates, very analogous in its composition to olivine. 

 We are tlius 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 demonstra- 

 tion 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 mag- 

 nesia, or of both, would have been produced." 



The presence of these silicates in both our specimens is 

 shown by the gelatinous silica appearing ; yet a greater quan- 

 tity 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. 

 But it is needless to pursue the subject further with our pre- 

 sent limited knowledge of facts. 



Postscript. — Altered Dolomites. 



The preceding account was drawn up some months ago for 

 insertion in the Philosophical Magazine. On consideration, 

 however, it was thought best to withhold it until careful quan- 

 titative analyses of the limestones should be obtained. This 

 has now been accomplished through the kindness of Dr. Ro- 

 bert D. Thomson, of Glasgow University, of whose skill and 

 competence it would be presumption in me to speak. 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 kindly furnished me by Dr. Robert D. Thom- 

 son are as follows : — 



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. 



No. 4 is the altered limestone from contact with the over- 

 lying trap at Ascog Mill ; it is an impure dark-coloured rock 

 of an earthy aspect, and very like the trap which rests upon it. 



