THE PARALLEL ROADS OF GLEN ROT. 15? 
learned as a geologist, but obviously fitted by nature to 
grapple with her facts and to put them in their proper set- 
ting. I refer to Sir Thomas Dick-Lauder, who presented 
to the Royal Society of Edinburgh, on the 3d of March, 
1818, his paper on the Parallel Roads of Glen Roy. In y 
looking over the literature of this subject, which is now 
copious, it is interesting to observe the differentiation of gj.^ 
minds, and to single out those who went by a kind of 
instinct to the core of the question, from those who erred " tJV . 
in it, or who learnedly occupied themselves with its anal- 
ogies, adjuncts, and details. There is no man, in my 
opinion, connected with the history of the subject, who 
has shown, in relation to it, this spirit of penetration, this 
force of scientific insight, more conspicuously than Sir 
Thomas Dick-Lauder. Two distinct mental processes are 
involved in the treatment of such a question. Firstly, the 
faithful and sufficient observation of the data; and secondly, 
that higher mental process in which the constructive im- 
agination comes into play, connecting the separate facts of 
observation witli their common cause, and weaving them 
into an organic whole. In neither of these requirements 
did Sir Thomas Dick-Lauder fail. 
Adjacent to Glen Roy is a valley called Glen Gluoy, /^ . 
along the sides of which ran a single shelf, or terrace, , 
formed obviously in the same manner as the parallel roads 
of Glen Roy. The two shelves on the opposing sides of the 
glen were at precisely the same level, and Dick-Lauder 
wished to see whether, and how, they became united at the 
head of the glen. He followed the shelves into the recesses 
of the mountains. The bottom of the valley, as it rose, 
came ever nearer to them, until finally, at the head of Glen 
Gluoy, he reached a col, or watershed, of precisely the same 
elevation as the road which swept round the glen. 
The correct height of this col is 1,170 feet above the 
sea; that is to say, 20 feet above the highest road in 
Glen Roy. 
From this col a lateral branch-valley Glen Turrit led 
down to Glen Roy. Our explorer descended from the col 
to the highest road of the latter glen, and pursued it exactly 
as he had pursued the road in Glen Gluoy. For a time it 
belted the mountain sides at a considerable height above 
the bottom of the valley; but this rose as he proceeded, 
coming ever nearer to the highest shelf, until finally he 
