160 *& A GMEXTS V SCIENCE. 
Here w6 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-Lauder. His solution was this: Taking all 
their features into account, he was convinced that water 
only could have produced the ten-aces. But how had the 
water been collected? He saw clearly that, supposing the 
mouth 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 Roy. 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 reached 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- 
