Mr. McCombie Stewart (a man of diverse talents) confirms the strange qualities of the largest stones:
Mr. McCombie Stewart, the station-master at Dyce, who should be consulted by any one visiting Dyce for scientific purposes, informed me that there was formerly a hole in the middle of the circle, which might be suggestive of the former existence of a kist; he also told me that there was supposed to be iron in the largest stones, and this seems very probable, for, on working my rough plans out at home, I found a disagreement in the compass-bearings. In this emergency I applied to Mr. McCombie Stewart, sending him a plan and asking him to verify my compass-bearings and some other particulars. He was so kind as not only to do this, but to get one of the Engineers of the railway to make an exact plan of the circle, showing the bearing of each stone from the centre. I am happy to be able to say as showing the accuracy of my own methods, that my plan superposed upon his gave practically the same results.
In the letter accompanying the plan, Mr. McCombie Stewart, who is qualified to speak as a geologist, says, “We were unable to account for the peculiar ringing sound of the altar stone, unless it be caused by the flat shape of the stone, having its side firmly fixed in the ground, the projecting part having a certain vibration – or if it were from the hard heathen substance of an iron nature – but one thing is certain, the stone is not of the same nature as those belonging to the neighbouring quarry.
From p45 of
Stone Circles Near Aberdeen
A. L. Lewis
The Journal of the Anthropological Institute of Great Britain and Ireland, Vol. 17. (1888), pp. 44-57.