SKY. GEOLOGY. SILICEOUS SCHIST. 359 



proximity of the trap rock. The alternation of the two 

 substances here described, which have doubtless been 

 originally shale and sandstone, similar to the unchanged 

 specimens occurring 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 

 its present form, also converted the shale into Lydian 

 stone. The position of every specimen of siliceous 

 schist or Lydian stone which I have seen in Scotland 

 is analogous to this. In Cruachan, in Rasay, in the 

 Shiant isles, at Talisker, and in Scalpa, it forms beds, 

 in contact with and involved either in granite or in 

 trap, which, from their connexions and positions, appear 

 to have been common clay slate in those cases where 

 it belongs to the primary strata, and shale in those 

 where it forms a constituent of the secondary. It 

 is possible also that the grey varieties of this substance 

 may, in certain cases, have originated from common slate 

 clay, and the Lydian stone from black shale. 



Between this Lydian stone and fine grained basalt 

 there is often no assignable difference of character except 

 that of the large fracture, a circumstance perhaps neces- 

 sarily resulting from the unaltered stratification of the 

 former. Nor is there any reason to doubt this resem- 

 blance, since the same materials under a different form 

 probably compose both rocks. Chemical analysis un- 

 fortunately offers 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 is interesting 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 the 

 sandstone bordering on it. If basalt has been produced 

 by the fusion of beds of slate, the necessary analogy 



