the fine grained basalts, particularly such as I have mentioned as occurring in the form of veins in the Cuchullin. The sandstone laminæ possess at the same time the hardness and jaspideous aspect of that which I formerly described[1] as lying in contact with the greenstone of Stirling castle. Considering therefore the analogy of these two sand-stones both in aspect and position, we may fairly conclude that they have in these instances been altered from their original texture, in consequence of the proximity of the trap rock. The alternation of the two substances here described, which have doubtless been originally shale and sandstone similar to that of the unchanged specimens which we meet with in various parts of this shore, gives us an equal right to conclude that the same influence of the neighbouring trap which converted the sandstone into hornstone, also converted the shale into Lydian stone. This in fact is the position of every specimen of siliceous schist or Lydian stone which I have seen in Scotland. In Cruachan, in Raasa, in Shiant, at Talisker, it forms beds, in contact with and involved in trap, which, from their connections and positions, appear to have been common clay slate, in those cases where it belongs to the primitive strata, and shale in those where it has formed a constituent of the secondary ones. It is possible also that the gray varieties of this substance may have originated from slate, and the black or Lydian stone from shale: additional facts however would be required to prove this part of the theory.
I have said that the resemblance between this Lydian stone and fine grained basalt is so perfect that there is no assignable difference of character save that of the large fracture, a circumstance of difference perhaps necessarily resulting from the unaltered stratification
- ↑ Geol. Trans. vol. 2.
