210 FKAGMENTS OF SCIENCE. 



gradually rise, until it had reached the level of the 

 col which divides Glen Roy fiom 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 remained 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 

 removed from the mouth of Glen Roy as to establish a 

 connection 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 

 question next occurs, What was the character of the 

 assumed barrier which stopped the glens ? There are 

 at the present moment vast masses of detritus in certain 

 portions of Glen Spean, and of such detritus Sir 

 Thomas Dick-Lauder imagined his barriers to have 

 been formed. By some unknown convulsion, this 

 detritus had been heaped up. But, once given, and 

 once granted that it was subsequently removed in the 

 manner indicated, the single road of Glen Gluoy and 



