160 FRAGMENTS OF SCIENCE. 



Here we. have a series of facts of obvious significance as 

 regards the solution of this problem. The effort of the 

 mind to form a coherent image from such facts may be 

 compared with the effort of the eyes to cause the pictures 

 of a stereoscope to coalesce. For a time we exercise a 

 certain strain, the object remaining vague and indistinct. 

 Suddenly its various parts seem to run together, the object 

 starting forth in clear and definite relief. Such, I take it, 

 was the effect of his ponderings upon the mind of Sir 

 Thomas Dick-Lander. His solution was this: Taking all 

 their features into account, he was convinced that water 

 only could have produced the terraces. But how had the 

 water been collected? He saw clearly that, supposing the 

 month of Glen Gluoy to be stopped by a barrier sufficiently 

 high, if the waters from the mountains flanking the glen 

 were allowed to collect, they would form behind the 

 barrier a lake, the surface of which would gradually rise 

 until it reached the level of the col at the head of the glen. 

 The rising would then cease; the superfluous water of Glen 

 Gluoy discharging itself over the col into Glen Boy. As 

 long as the barrier stopping the mouth of Glen Gluoy con- 

 tinued high enough, we should have in that glen a lake at 

 the precise level of 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 Roy 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 readied the level of the col which divides Glen Roy 

 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 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 oc- 



