ROCKS OF THE NORTHERN HIGHLANDS. 409 



1. Loch- Ailsh Group. 



The prevailing rock in this mass is reddish in colour, of a grani- 

 toid texture, and mainly composed of quartz and felspar. Prof. 

 Bonney describes it (No. 109, p. 420) as " a kind of syenite ;" but 

 hornblende is certainly not a very conspicuous constituent. An ex- 

 ceptional variety, garnetiferous (No. 72, p. 420), occurs east of Loch 

 Borrolan, on the slope north of the road. 



These rocks form the ranges of hills which strikes S.E. and N.W., 

 between the gap south of Loch Ailsh and Ledmore, and culminates 

 in Cnoc-na-Strone, above Ledmore. From the hill west of the gap, 

 the igneous rock turns to the ISLE., and appears here and there through 

 the quartzite of Loch Ailsh and the south-eastern slopes of Scounan 

 More. Growing more conspicuous, it rises into Scounan Beg, a pro- 

 minent hump, recognizable at a distance by its coppery colour. The 

 igneous rock is here in immediate contact with the old gneiss of Scou- 

 nan More, and is intrusive in it in a small mass 100 yards to the west 

 of Scounan Beg. Vertical jointing is very conspicuous in the latter 

 hill. The syenite is continued across the Oykel, being well exposed 

 in the river-bed for a breadth of nearly two miles, its eastern boun- 

 dary passing a little north of Kinloch Ailsh. Its north-eastern ex- 

 tremity forms a reddish mass, the counterpart, on the opposite side 

 of the river, of Scounan Beg. A little east of this point it comes 

 up to the eastern gneiss, cutting out the marble. All other con- 

 spicuous elevations which have been marked as " igneous " in this 

 area are Hebridean. 



2. Igneous Mock of the Quartzite. 



I have already referred to the occurrence of this rock in the 

 Quartzite as a useful indication of a certain horizon. On the 

 south-west side of Loch Assynt the " porphyry " contains very large 

 crystals of felspar. This interesting variety has been described 

 by Prof. Heddle*. Generally the igneous rock is more compact, 

 and might be described as a porphyritic felsite. It is frequently in- 

 terbedded, but is probably not contemporaneous. Diorite (No. 71, 

 p. 419) also frequently occurs at the same horizon. 



3. Granite of Durness and Loch Erriboll. 



The granite of Loch Erriboll is mainly composed of quartz and 

 a brick-red felspar, with a peculiar glassy cleavage, which is charac- 

 teristic of the granite east of the loch. The granite intrusive in the 

 Hebridean of Durness is of similar composition ; but the distinctive 

 cleavage is absent. Veins are much more abundant in the Arna- 

 boll gneiss than in the Hebridean of this district. They rarely 

 appear in the Hope series in this area ; but I noticed in Craig-na- 

 faolin, where the Arnaboll passes conformably up into the higher 

 group, that some veins penetrated the lower beds of the latter. 



* Mineral. Mag. Aug. 1881, p. 233. 



