378 
POPULAR SCIENCE REVIEW. 
scope 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 ponder- 
ings upon the mind of Sir Thomas Dick-Lauder. His solution 
was this : Taking all their features into account, he was con- 
vinced that water only could have produced the terraces. He 
saw clearly that, supposing the mouth of Glen Gluoy to be 
stopped by a barrier, if the water from the mountains flanking 
the glen were allowed to collect, it 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 Eoy. As long as the 
barrier stopping the mouth of Glen Gluoy continued, 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 actually form the shelf revealed by observation. 
So much for Glen Gluoy. But suppose the mouth of Glen 
Eoy also stopped by a barrier sufficiently high. Behind it, 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 Eoy from Glen 
Spey. Here the rising of the lake would cease ; its superabun- 
dant water being poured over the col into the valley of the 
Spey. This state of things would continue as long as the 
barrier remained at the mouth of Glen Eoy. 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 over- 
spreading the mountains, and would form the highest road or 
terrace of Glen Eoy. 
And now let us suppose the barrier to be so far removed 
from the mouth of Glen Eoy as to establish a connection between 
it and the upper part of Glen Spean, while the lower part 
of the latter glen continued blocked up. Upper Glen Spean 
and Glen Eoy 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 Eoy would 
sink from the level it had previously maintained, to the level of 
its new place of escape. This new lake-surface would corres- 
pond exactly with the lowest parallel road, and it would form 
that road by its action upon the drift of the adjacent moun- 
tains. 
In presence of the observed facts, this solution commends 
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 
