70 Dr. Mac Culloch's Sketch of the 



eties of syenite which will be described hereafter, while the dark 

 ones, with the exception of some greenstone, seem to be every 

 where formed of clinkstone. The tendency of these syenitic 

 substances to decomposition, explains the weathered appearance 

 of the mountains which I have characterized by the name of 

 the Red Hills, and the accumulation of the fragments which 

 result from it, accounts for the roundness and tameness of their 

 outline. These causes very generally impede the view of the 

 natural rock. Wherever it can be seen it appears to possess the 

 disposition which is I believe invariably characteristic of this class of 

 rocks, forming large irregular beds, much rifted and fissured, and 

 never continuous in inclination and extent for any considerable space. 



It might be deemed important to determine here the nature of 

 the junction between the dark clinkstone and the light coloured 

 syenite, but if it has not been discovered here, it is fortunately visible 

 in other places, while at the same time the community of these two 

 rocks, in almost every circumstance but colour, seems to bespeak a 

 common position. Ben-y-chat in Mull, and many situations in 

 Arran, present examples of this connection, nay in some cases of a 

 perfect transition. The phenomena visible in Rum, in St. Kilda, 

 and in others of the Western islands coninclde with these, and 

 offer a proof which may by a fair analogy be extended to Sky, 

 that there is a community of geological position in these two 

 rocks. 



I have much more reason to regret the Insuperable difficulties 

 which I experienced in attempting to trace the connection between 

 the syenite and the stratified rocks, the sandstones and limestones 

 which were described in the commencement of this paper. 



The only real contact of this substance with those strata is to be 

 seen at the marble limestone in Strath, and I have traced it in the 



