SKY. GEOLOGY. OVEKLYING ROCKS. 367 



appeared during the examination of the limestone ; namely, 

 that the syenitic boundary is very irregular, and is accom- 

 panied by detached portions of the calcareous rock. 



As the boundary of the syenite is found principally 

 in the tract occupied by these strata, there is little to 

 be said respecting its relations to the red sandstone j 

 since no opportunity offers of investigating these. The 

 junction may possibly be found near Loch Sligachan, 

 where that rock occurs ; and it is equally probable that 

 it exists somewhere in the hills near the southern boundary 

 of the limestone, since the syenite appears there to cover 

 the line where the one set of strata succeeds to the other. 

 I had not the good fortune to find it, the hills being 

 much covered with soil and vegetation; but there is no 

 reason to expect any appearances different from those 

 which happen in the case of analogous junctions, or from 

 those already enumerated as occurring in the superior 

 strata. 



It is now necessary to inquire into the connexion of 

 the syenite with the other unstratified substances. As 

 ah 1 these appear to bear a common relation to the strati- 

 fied rocks, so they seem to possess a community of 

 position and connexion among each other. In some 

 instances there is a mere gradation ; the exclusion of one 

 of the component parts giving rise to a new rock, which 

 in return receiving an addition of some other material, 

 becomes a third ; no variation of the great features taking 

 place except that which arises from different degrees 

 of durability, or from those changes of the bedded to 

 the amorphous, or to the columnar forms, which equally 

 occur where there is no change of substance. In other 

 cases there is a decided line of separation, at times con- 

 nected with a considerable change of composition, at 

 others marked by that change of colour arising from the 

 state or proportions of the iron which, though strictly 

 speaking a change of composition also, is not a radical 

 or important one. In these latter cases the rocks seem 



