20 ME. T. F. JAMIESON SUPPLEMElfTARY 



the mouth of Glen Spean, and the waterworn pebbles, I think, are 

 more plentiful than even at the other locality. 



The great accumulation of gravel and pebbly debris at Inverlaire 

 and the eotrance to Loch Treig must have been caused, not by the 

 action of the lake, but by the glaciers. Probably much of it was 

 ground out of the bottom of Loch Treig by the sole of the glacier 

 which occupied that basin. This heavy mass of debris forms another 

 good link in the chain of evidence in favour of the glacial theory, 

 for how otherwise can it be accounted for at this particular spot ? 



During strong westerly gales the wind sweeps with great fury 

 through the narrow rocky pass at the head of Glen Roy, as I know 

 from personal experience, and here I remarked that the rocky emin- 

 ences bore evidence of water-wearing about the level of the two 

 higher lines (the lowest one does not extend so far up), while there 

 are also some waterworn pebbles on them. This observation is 

 confirmed by Dr. Tyndall, who says : " Near the head of Glen Eoy 

 the highest ' road ' ceases to have any width, for it runs along the 

 face of a rock, the effect of the lapping of the water on the more 

 friable portions being perfectly distinct to this hour." ^ 



Apart from the deltas these instances are sufficient to show that 

 the lake must have existed for a considerable time, for the broad 

 shelves of detritus near Leek B,oy, with their waterworn pebbles, 

 imply a lengthened action of water, and so does this waterworn rock 

 at the head of the glen. 



XI. POEMATION OF THE TeEEAGES. ActION OF THE LaKE-MAEGIN. 



As regards the exact mode of operation by which the terraces 

 were formed along the hillsides, this is a point which, I confess, did 

 not engage much of my attention, and what I did say about it 

 requires some correction. Of all observers. Sir John Lubbock^ 

 seems to me to have treated this matter with most acumen. 



In considering the form and dimensions of the ' roads ' at any 

 particular spot three elements claim our attention, viz. : — 



1. The depth or vertical measurement. 



2. The breadth or horizontal measurement. 



3. The angle of inclination to the horizon. 



The vertical extent seems to represent the range of wave-action, as 

 Sir John Lubbock points out. Accordingly we find it greatest in 

 those places which are most exposed to the wind. It marks the 

 " zone of agitation '' in the water. As the Glen Eoy lake was 

 narrow and shut in by hills, its surface would not be agitated to 

 any great depth unless in a few places, chiefly near the ends : the 

 action of the waves would therefore be comparatively feeble. The 

 smallness of the vertical measurement affords another good argu- 

 ment that the lines are not sea-beaches, for if so their vertical 



^ Proc. Eoy. Inst. vol. viii. (1876) p. 240. 



'"■ Quart. Journ. Geol. Soc. vol. xxiv. (1868) p. 83. 



