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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Dec. 5, 



bold cliff on the gneiss (a), which forms Ben Stack and the moun- 

 tainous region to the west coast. Beyond the Lodge, granulite like 

 that on Loch Emboli breaks up through the quartzite, and may be 

 traced for more than a mile along the upper part of the mountain, 

 where it is free from the detritus that covers the slopes near the 

 road and loch. Mr. Cunningham represents this rock as gneiss* ; 

 but it is truly unstratified, and its intrusive character is shown by 

 the quartzite having been pushed up and resting on it in broken 

 and nearly vertical beds, dipping 85°, to 40° "W. Higher up the 

 lake this igneous rock meets the gneiss of Ben Leick, exposed in 

 lofty vertical cliffs, and dipping first 20°, to E. 75° N., and then 

 15°, to E. 30° rJ. On the south side of the ridge, towards Strath- 

 na-carrian, the gneiss, full of igneous veins and greatly contorted, 

 has quite the aspect of the gneiss round Loch Inver and Scourie, but 

 dips at 40°, to E. 20° S. It seems merely the continuation of the 

 beds seen in the west part of the section, in Ben Stack. That there 

 is in this place no conformable upward succession from quartzite to 

 gneiss is proved not merely by the clear break in the section, but 

 even more by the quartzite, which, on the north side of Loch Stack 

 and Loch More, has a thickness of 800 to 1000 feet, as well seen in 

 the front of Arkle and Foinaven, — on the south side of these lakes 

 disappearing, except a few fragments not the tenth part of that 

 thickness. This is the necessary result of denudation over a line of 

 fault ; whereas no amount of denudation could ever show less than 

 the full thickness of the deposits in any section of conformable 

 strata. As shown in the figure, I have been unable to detect any 

 trace of quartzite or limestone on the east side of the intrusive 

 rock. 



Lochs Glen Coul and Glen Dhu, and Glasven. — The quartzite, 

 with the same degraded dimensions, ranges across to Lochs Glen 

 Dhu and Glen Coidf, and in the rugged mountains tbat surround 

 the inner recesses of these noble sea-lochs is said to be again over- 

 lain by gneiss. This, however, is a mere optical deception, caused 

 by the rocks being seen from the low ground or the sea ; as the 

 quartzite, though dipping eastwards, only abuts on the rounded 

 knolls that rise up behind, and is generally separated from them by 

 a low marshy valley. The rock too, described as conformably over- 

 lying gneiss, is in some places an intrusive syenite, in others true 

 granitic gneiss, but rising up in nearly vertical masses with a strike 

 at right angles to the beds on which it has been said to rest. This 



* It seems to be the rock thus described : " Another prominent variety [of 

 gneiss] exists in a rock almost entirely composed of compact felspar and quartz, 

 arranged, not in distinct concretions, but, on the contrary, so closely connected 

 that their linear position can only be detected after atmospherical agents have 

 partially abraded the felspar. This remarkable gneiss forms some hills on the 

 south side of Loch More." — Cunningham, Geog. of Sutherland, p. 77. 



t In the geological map published in No. 62 of the Quart. Journ. Geol. Soc. 

 (May 1860), a broad band of ''upper quartz-rock " is laid down in this region. 

 It is rather remarkable that these same rocks, now described as quartz-rock, were 

 quoted as a true overlying gneiss both by Macculloch and Cunningham. As 

 stated in the text, the rook is gneiss (but not overlying) or a syenite. 



