Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/78

 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 tamemess 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 Sr. Kilda, and in others of the Western islands coincide 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