216 P. Macnair — Altered Clastic Rocks of Scottish Highlands. 



These are well exposed in the upper gorge of the Pass of Leny, and 

 at the foot of Loch Lubnaig. At the upper part of the Pass, 

 and near the foot of Loch Lubnaig, the greywackes become highly 

 gritty and conglomeratic, the pebbles being composed of quartz and 

 felspar set in a matrix of fine argillaceous material. Along the 

 eastern shores of Loch Lubnaig these greywackes are followed 

 by zones of slates, approaching more closely to a phyllite than 

 any we have seen in this group. They are well exposed in the 

 quarries on the side of the hill, and are shown still dipping, with an 

 average dip of about 50° to the north-west. Passing northwards, 

 they are seen to plunge under the massive beds of grit and 

 conglomerate shown on the south side of Loch Earn and in 

 Ben Vorlich, the position of which to the middle arenaceous zone 

 is shown in the section from Craig Na Challeich to Loch Earn 

 (Fig. 2). 



XII. Conclusion. 



It now remains for us, in conclusion, to make some general 

 remarks upon the structure, succession, and age of these rocks along 

 the margin of the Highlands. The sections we have described from 

 Perthshire have their beds dipping at such low and gentle angles 

 that little doubt can exist as to their exhibiting the true 

 succession of the beds. Sir A. Geikie notes this fact thus: "Over 

 many square miles the angles of inclination are low, and the 

 successive bands may be traced from hill to hill across strath and 

 glen, forming escarpments along the slopes and outliers on the 

 summits precisely as gently undulating beds of sandstone and 

 limestone may be seen to do in the dales of Yorkshire." 1 In 

 Perthshire and to the north-east, in the counties of Forfar and 

 Kincardine, along the Highland border, the order of succession 

 we have just given seems to hold good so far as we have had 

 opportunities of observing ; while on the other hand, to the south- 

 west of Perthshire and in the counties of Argyll and Dumbarton, the 

 rocks of the southern margin of the Highlands have been plicated 

 into a deep and acute synclinal fold with a reversal of dip ; this 

 reversal being well shown in the Ben Lomond section, where from 

 the clay-slates on the margin of the Highlands, and inward towards 

 Ben Lomond, the whole series of grits, slates, and greywackes are 

 seen to dip to the south-east. Thus our conclusion as to the true 

 succession of these rocks is exactly the opposite to that arrived 

 at by Prof. Nicol. 



Turning now to the difficult subject as to the age of these rocks, 

 little can yet be said as regards their true position in the geological 

 scale. The discovery of radiolarian cherts associated with the 

 graphitic schists of our upper argillaceous zone, according to Sir A. 

 Geikie, seems to point to the identity of these higher rocks with the 

 Ordovician of the southern uplands ; while our annelid tubes occurring 

 near the base of that zone might point to the similarity of it to 

 the Cambrian quartzites of the north-west of Sutherlandshire — thus 



1 Quart. Journ. Geol. Soc, Presidential Address, vol. xlvii, p. 73. 



