THE PARALLEL ROADS OF GLEN ROY 223 



its shelf, which lake, acting upon the loose drift of the 

 flanking mountains, would form the shelf revealed by 

 observation. 



So much for Glen Gluoy. But suppose the mouth of 

 Glen Eoy also stopped by a similar barrier. Behind it 

 also the water from the adjacent mountains would collect. 

 The surface of the lake thus formed would gradually rise, 

 until it had reached the level of the col which divides 

 Glen Koy from Glen Spey. Here the rising of the lake 

 would cease; its superabundant water being poured over 

 the col into the valley of the Spey. This state of things 

 would continue as long as a sufficiently high barrier re- 

 mained at the mouth of Glen Roy. The lake thus dammed 

 in, with its surface at the level of the highest parallel 

 road, would act, as in Glen Gluoy, upon the friable drift 

 overspreading the mountains, and would form the highest 

 road or terrace of Glen Roy. 



And now let us suppose the barrier to be so far re- 

 moved from the mouth of Glen Roy as to establish a con- 

 nection between it and the upper part of Glen Spean, 

 while the lower part of the latter glen still continued to 

 be blocked up. Upper Glen Spean and Glen Roy would 

 then be occupied by a continuous lake, the level of which 

 would obviously be determined by the col at the head of 

 Loch Laggan. The water in Glen Roy would sink from 

 the level it had previously maintained to the level of 

 its new place of escape. This new lake-surface would 

 correspond exactly with the lowest parallel road, and it 

 would form that road by its action upon the drift of the 

 adjacent mountains. 



In presence of the observed facts, this solution com- 

 mends itself strongly to the scientific mind. The ques- 



