174 



PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 26, 



about 2100 feet above the sea, or 1250 feet above the level of the line, 

 and, say, 1600 feet above the bottom of the vaUey. Now I found 

 the flank of this Craig Dhu strongly impressed with marks of glacial 

 action up to within a hundred yards or so of the very top. Rounded 

 shoulders of rock, scored and fluted horizontally, sometimes even 

 polished, may be seen in many places all over the side of the hill ; and 

 it is worthy of notice that these appearances are well displayed 

 immediately above, below, and even on the very line itself. The 

 highest well-marked scores observed by me were at a level of 300 

 feet below the top of the hiU, or (say) 1300 feet above the bottom of 

 the valley ; but transported boulders (blocs perches) occur up to near 

 the very summit. From the brow of Craig Dhu to the brow of Ben 

 Chlinaig, on the opposite side of Glen Spean, is a distance of two miles 



Fig. 3. — Outline-section across Glen Spean. 



Ben 

 Chlinaig. 



Eiver Spean 



Craig Dhu, 

 2100 feet. 



Col at the head 



ofGlenGlaster, 



1060 feet. 



or SO ; here, therefore, is a striking proof of what a volume of ice 

 must have swept down this valley, if these scores were caused, as I 

 believe they were, by this agency. 



The rock of the hiU, wherever I saw it, consists of micaceous 

 gneiss or mica-schist, dipping N.W. at a very high angle, with some 

 thin dykes or beds of felspar-porphyry. Now, the cropping out of 

 the ragged edges of the gneiss-strata obliquely to the east must 

 have afibrded tough morsels for the bite of even a glacier-stream 

 moving westward ; but notwithstanding this disadvantageous circum- 

 stance of the edges being presented towards, and their backs away 

 from the stream, it became evident to me, after a careful examina- 

 tion of much of the hill aU along its south flank, that from top to 

 bottom the scoring agent had moved from east to west. This was 

 apparent not only from the greater rounding and polishing of the 

 east faces, but a further proof was aiforded by the movement of rock- 

 masses : for instance, from a great dyke of white quartz a large 

 angular block had been torn off and carried some yards to the west- 

 ward ; also porphyry fragments indicated a similar direction of trans- 

 port. But there was another most striking and convincing proof of 

 this nature. The gneiss over the hiU-top being quite bare, or covered 

 only by a thin peel of turf, it was easily seen that no other rock 

 was present ; it will therefore be admitted to be a highly interesting 

 fact when I state that large angular boulders of syenitic granite, 

 precisely similar in mineral quality to the low mass of that rock 

 which occurs in situ in the bottom of the vaUey to the eastward, 

 opposite the gorge of Loch Treig, are scattered in great numbers all 

 over the brow of the hill, resting on the bare upturned edges of the 

 gneiss, which is shorn and rounded into smooth outlines ; and what 



