362 PROCEEDINGS OE THE GEOLOGICAX SOCIETY. [Feb. 15, 



vation to the sheltered position it occupies in the bosom of the hill 

 between two projecting buttresses of rock. Not a trace of drift re- 

 mains on the slopes of mica-slate on the opposite side of the river ; 

 all has been swept away. 



Between Loch Tummel and the valley of the Tay, there lies a great 

 ridge of mica- slate stretching in an east and west direction, with 

 numerous peaks reaching from 2000 to 2600 feet high. The drift 

 on the northern slope of this ridge stretches up to a height of 

 1500 feet. 



Fig. 11. — Section across the Ridge of Mica-slate, showing the Drift 

 upon its Northern Slope opposite Bonslceid. 



I, Ordnance station (2015 ft.). 2. Boulders. 3. Boulder-drift. 4. Water-rolled gravel. 

 5. The River Tummel. 



I descended the course of a small rivulet that joins the Tummel 

 opposite the House of Bonskeid, and which has cut through the earthy 

 covering in such a manner as to afford a good view of its structure. 

 Having an aneroid with me, I took measurements as I went along, 

 and found that the upper limit of the drift lay at an altitude of about 

 1520 feet. At 1494 feet it was 16 feet deep, resting on a smoothed 

 rock-surface showing grooves and striae pointing N. 20° to 30° W. 

 At 1424 feet the thickness had increased to 30 feet; and at 1005 feet 

 the drift reached a depth of 50 to 80 feet, resting on coarse gneiss, 

 showing parallel furrows and scratches pointing N. 70° W. It con- 

 tinued of considerable thickness for some distance further down; but 

 below 700 feet it almost entirely disappeared, as if it had been washed 

 away; and at 612 feet the loose water-rolled gravel made its appear- 

 ance, resting on some denuded remains of the drift, of which there 

 was a patch left some ten feet thick. This was 185 feet above the 

 bed of the Tummel at the mouth of the rivulet. 



It will be remembered that the drift on the flank of Meal Uaine 

 was cut away at a similar height, and that water-rolled gravel made 

 its appearance there also a little beneath its lower lirnit. In these 

 instances, therefore, I think we have evidence that the drift has been 

 subjected to great denudation, and that in the water-rolled gravel we 

 have traces of some of the forces that effected it. 



§ 3. At an elevation of 1485 feet, in a small rocky hollow in the 

 course of this stream, where the drift had mostly been removed, I 

 found the vertical sides of the rock fluted, grooved, and scratched 

 almost horizontally, the markings pointing due north. 



