S8S Professor Agassiz on the Glacier Theory, 



out the influence of a cause which would, at the same time, 

 have occasioned the disappeai'ance of terraces having so little 

 consistence ; whereas, in a country which presents so many 

 traces of ancient glaciers, the supposition of a great glacier, 

 descending from Ben Nevis, and shutting up the valley of the 

 Spean, by resting on Moeldhu, which is opposite, combined 

 with the influence of a glacier issuing from Loch Treig, and 

 which would bar the valley a second time at that height, 

 would explain all the facts. The glacier of Loch Treig, of 

 inferior size to that of Ben Nevis, would, first of all, be low- 

 ered at two difi'erent times, after having for a certain period 

 maintained the water contained between the two glaciers at 

 the level of the two upper terraces. During these two lower- 

 ■ings, the waters would run to the east, proceeding by the val- 

 ley of the Spey, owing to the inconsiderable height of the col 

 which separates that valley from Glen Spean. Whenever the 

 glacier of Loch Treig disappeared completely, the water would 

 be able to extend to the end of Glen Spean, and likewise in- 

 vade Loch Treig ; which explains the continuity of the lower 

 terrace, while the two upper ones terminate abruptly opposite 

 Loch Treig. Afterwards, when the great glacier of Ben 

 Nevis no longer reached Moeldhu, the waters would run to 

 the west, and water would remain only in the hollows which 

 are now occupied by Loch Treig and Loch Laggan. The 

 sudden termination of the three terraces, on the two sides of 

 Glen Spean near the Bridge of Roy, will likewise be under- 

 stood from this explanation. The supposition now made is 

 confirmed by a fact which there is no other mode of account- 

 ing for : viz. that the bottom of Glen Spean in front of Loch 

 Treig is not only polished with that polish characteristic of 

 glaciers, but is moreover scratched transversely, that is to say, 

 at right angles to the direction of the valley, by a cause which 

 evidently proceeded from Loch Treig. I do not believe that 

 a locality exists, where the facts indicate, in a more special 

 manner, the cause which has produced them. The horizontal 

 terrace of Glen Gloy is susceptible of a very natural explana- 

 tion by a glacier issuing from the valley of Loch Arkeig, 

 crossing Loch Lochy, and damming up Glen Gloy above Low 

 Bridge. This supposition would also clear up the diff^erence 



