Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/108

 of the Lydian stone. Nor is there any reason to doubt this resemblance, 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 to inquire by what power the vicinity of trap operates in influencing 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 necessary analogy between the Lydian stone and basalt will appear conspicuous, 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 contain 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 described, 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 economical uses to which it seems applicable.