My Dear Sir,

I have just lighted upon what I consider an important fact with reference to the parallel roads of Glen Roy & w^h. may probably interest you.¹

In the letter I wrote you after my return from Lochaber I think I mentioned that owing to the impossibility of tracing the lowermost line along the shores of Loch Laggan with any degree of certainty I felt somewhat in doubt as to the fact of the outlet at Makoul coinciding with that line—² I afterwards wrote to both M^r. Chambers & M^r. Milne Home on the subject but failed to elicit anything certain on the subject.³

I had however little doubt in my own mind that Loch Laggan had formerly discharged its waters E,ward by that channel from the circumstance of the great old delta of the Gulban which fills the S.W. corner of the lake & rises high above its present level.

—But by the greatest good fortune I find that the ordnance survey had run a line of spirit levelling up Glen Spean on to Dalwhinnie. and from the record of the engineers it appears that the water shed at Makoul just coincides with the lowermost of the Glen Roy lines— Thus, the height of said line found by the spirit levelling of an engineer employed for M^r. R. Chambers is 847 feet above the sea   it is not stated whether this refers to high water or mean sea level, but most of these surveyors usually take high water mark. and the summit level of the road at the water-shed of Makoul according to the Ordnance surveyors is 850 feet above the mean sea level.

—Now if the sea level taken by M^r. Chambers’ surveyor was high water mark it w^d. make the Glen Roy line up somewhat over 850 feet— something also would depend upon what part of the line the surveyor took as the line of terrace is generally several feet in vertical height.

Anyhow the coincidence is too close to be accidental—

I am inclined to think however that the level of the outflowing water at Makoul had been at one time fully 860 feet. judging from some measurements w^h. I made of the height of the old delta at the S. W. corner of L. Laggan. & also from my remembrance of the beds of waterworn pebbles &c at the Makoul outlet w^h. rise some feet I think above the summit level of the road. —Whether this is owing to some inequality in the level of the land that has occurred since the period of the Glen Roy roads—or whether it is owing to some discrepancy in the levellings I cannot say—

But I think it is evident that over a stretch of 20 miles from E. to W. this lowermost of the Glen Roy lines is wonderfully near if not altogether parallel to the present sea level, and I think there can be now very little doubt that it coincides with this outlet & has been determined by it.

If you think Sir C. Lyell w^d. be interested in this communication I should feel much obliged if you w^d. show it him as it w^d. save me writing it over again.⁴

I am | My dear Sir | Your very obed serv^t | Tho^s. F. Jamieson

C. Darwin Esq | F. R. S | &c &c &c