150 



The Glacial Theory. 



height above the bottom of the valley depends on the point of obser- 

 vation. The second is 212 feet above the first, and the third 82 feet 

 above the second. It is to be remarked, that the two upper terraces 

 make the round of Glen Roy, whereas, in Glen Spean they do not 

 extend higher than the opening of the valley of Loch Treig. I no- 

 ticed them on the left side of Glen Spean between Loch Treig 

 and the Bridge of Roy, as well as on the flanks of Glen Roy ; and I 

 mention this particularly, because they are not indicated at that 

 point in the maps which represent their position. It is evident that 

 these terraces indicate levels of water. The next enquiry is, if the 

 barriers which restrained these lakes have disappeared, or if the valley 

 has been elevated at different times above the level of the water ? 

 The perfect horizontahty of these terraces, at three different levels, 

 appears to me irreconcilable with the idea of a repeated soulevement 

 of the surface. The ablation of a rocky barrier seems impossible 

 wdthout the influence of a cause which would, at the same time, have 

 occasioned the disappearance 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 Moel- 

 dhu, which is opposite, combined with the influence of a glacier issu- 

 ing from Loch Treig, andwhich 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 lowered 

 at two diff'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 lowerings, the waters would 

 run to the east, proceeding by the valley 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 invade Loch Treig ; which explains the continu- 

 ity 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 



