622 D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER. 
blockage would thus be subject to the eroding action of the sea, as well as to 
that of streams issuing from the adjoining hills. 
IV. Having in the foregoing part of this memoir explained my own theory, 
and noticed some of the objections to it, I refer next to the other solutions of 
the problem which have been suggested. 
Darwin's Marine Theory. 
1. From what I have said regarding the prevalence of the sea over the 
highland hills, and the deposit of detritus, by means of sea currents, it might 
be supposed that I could scarcely refuse assent to Mr DaArwin’s views. But 
whilst I admit that there are traces of the sea at great heights on our coasts 
and mountains, it is not inconsistent to hold that whilst the sea was subsiding, 
lakes would be formed in favourable districts. 
In the first place, let me state the grounds on which, as it seems to me, 
the Parallel Roads could not have been sea-beaches. 
The principal difficulty which Darwin and CHAMBERS had to contend with 
is the circumstance, that in each Glen there is proved to have been an old 72ver 
course, by which the waters of the lake flowed out to a lower level. . 
When Mr Darwin wrote his memoir, these old river courses had not been 
discovered. All that he knew was, that the shelves in each of the glens coin- 
cided with a col or summit level which separated the glen from some adjoining 
glen. Mr Darwin stated, that in the Pacific Ocean, there were many cases 
where arms of the sea were divided or separated by a spit of sand, which was 
thrown up by tides or storms. He assumed that the highest shelf in Glen — 
Roy was on the same level as the highest in Glen Gluoy, and that the sea may 
have stood at the same level in Strathspey. Mr Darwin, when he made his 
survey, laboured under two great disadvantages. He had no spirit-level with 
him, and he did not visit the Pass of Mukkoul, where there is the un- 
mistakable river channel, by which Loch Laggan discharged itself into Strath 
Spey. 
RosBEeRT CHAMBERS was not under these disadvantages. He and I had eacha 
spirit-level, as well as a barometer and a sympysometer, by means of which we 
satisfied ourselves that the highest shelf in Glen Roy was at least 12 feet 
below the highest shelf in Glen Gluoy. In fact, he was the first to detect the ap- 
pearance of an old river course between these two glens. In his “ Ancient Sea 
Margins,” he alludes to the joint survey which he and I made, and also to the 
Memoir read by me in this Society, and which appeared in our Transactions before 
“his book was published. He takes no notice of the facts stated in my Memoir 
regarding these old river courses, though so conclusive against the marine 
theory which he had adopted. He, however, oddly enough makes the following 
admission regarding one of the lakes :—‘‘ The Pass of Mukkul,” he says, “ the 

