Mineralogy of Sky. 99 



the fine grained basalts, particularly such as 1 have mentioned as occur- 

 ring in the form of veins in the Cuchullin. The sandstone lamince 

 possess at the same time the hardness and jaspideous aspect of that 

 which I formerly described* 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 tex- 

 ture, in consequence of the proximity of the trap rock. The alter- 

 nation 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 neigh- 

 bouring 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 differ- 

 ence of character save that of the large fracture, a circumstance of 

 difference perhaps necessarily resulting from the unaltered stratifica- 



* Geol. Trans, vol. 2. 

 N 2 



