100 Dr. Mac Culloch's Sketch of the 



tion of the Lydian stone. Nor is there any reason to doubt this re- 

 semblance, since the same materials under a different form probably 

 compose both rocks. Chemical analysis unfortunately offers us no 

 temptation to try this analogy further, since the variable composition 

 of basalts as well as of schist, a variation necessarily arising from the 

 circumstances of their formation, would prevent the possibility of 

 comparing any two specimens even of the same substance. It may 

 be an interesting matter of speculation tq inquire by what power the 

 vicinity of trap operates in in-fluencing the change from shale to 

 Lydian stone, as well as in producing the much better known 

 changes which occur in sandstone bordering on trap. If basalts have 

 been in any case produced by the fusion of beds of slate, the neces- 

 sary analogy between the Lydian stone and basalt will appear conspi- 

 cuous, and we have only to consider it as a shale brought into 

 the state of basalt by fusion, without such further disturbance as to 

 destroy its original stratification. 



Although both the neighbouring sandstone and alternating shale 

 which have undergone no change from the vicinity of the trap con- 

 tain shells in considerable abundance, I did not perceive any marks 

 of them in this bed of Lydian stone. They may nevertheless exist, 

 although they escaped my observation. I have little doubt that the 

 instances of basalt containing shells which have so often been descri- 

 bed, have sometimes been cases analogous to this : the observers, 

 attending to the composition rather than to the disposition of the 

 rock, having easily been led into error from the perfect similarity of 

 the indurated shale to fine grained basalt. I may venture to point 

 out these instances as calling for re-examination. 



I shall terminate this account of the minerals found in Sky, by 

 a more detailed description of the marble of Strath, adverting to 

 those oeconomical uses to which it seems applicable. 



