282 SILICEOUS SCHIST. 



of which hornblende is a constituent part ; the analogy 

 of the respective causes being apparent, and serving 

 to add to the proofs already given, that the influence 

 of granite and porphyry in the one instance, has given 

 rise to those changes of character which, in the other, 

 have been produced by that of trap. 



Examples of siliceous schist in the situations now 

 described, are well known in the trap districts of Ireland. 

 In Scotland they are frequent ; but the most satis- 

 factory examples occur in the Shiant isles and in Sky. 

 In the district of Trotternish (Geol. West. Islands), the 

 gradual change from shale to Lydian stone, the dis- 

 tance to which the influence of the trap reaches, the 

 corresponding effects on the pure and on the impure 

 limestones and on the clays, are such as to afford a 

 demonstration of the causes, as perfect as if the same 

 substances had been exposed to the action of ordinary 

 heat. It is unnecessary to repeat the arguments which 

 these facts add to the igneous theory of trap : nor how 

 the occurrence of the same appearances in the same 

 kinds of rock when near granite, tend to confirm the 

 igneous origin of that rock also. 



It is plain that no general definition can comprise 

 substances so variable as those which are ranked under 

 this head. Even if divided into the two classes of 

 primary and secondary, the difficulty is not removed. 

 Though the prevailing varieties were described, the 

 knowledge of the rock in nature must still be derived 

 from the examination of its geological connexions, 

 from a comparison with all other rocks with which it 

 might be confounded, and, perhaps, from a certain 

 degree of that traditional information, conveyed by 

 authenticated specimens, without which the knowledge 

 of rocks and minerals will always be of difficult ac- 

 quisition. 



I may however, remark, that the primary siliceous 



