At the island called Barrahead,

...one of the Hebrides, a remarkable example occurred during a storm in January, 1836, in the movement of a block of stone, which, from measurement taken on the spot, is 9 feet x 8 feet x 7 feet=504 cubic feet, which, allowing 12 feet of this gneiss rock to the ton, will be about 42 tons weight.


This great mass was gradually moved 5 feet from the place where it lay, having been rocked to and fro by the waves till a piece broke off, which, rolling down and jamming itself between the moving mass and the shelving rock on which it rested, immediately stopped the oscillatory motion, and thus prevented the further advance of the stone.


Mr. Reid, the principal keeper of Barrahead Lighthouse, the assistant keeper, and all the inhabitants of the little island, were eye-witnesses of this curious exhibition of the force of the waves ; and Mr. Reid also gives the following description of the manner in which they acted upon the stone:


" The sea," he says, "when I saw it striking the stone, would wholly immerse or bury it out of sight, and the run extended up to the grass line above it, making a perpendicular rise of from 39 to 40 feet above the high-water level. On the incoming waves striking the stone, we could see this monstrous mass of upwards of 40 tons weight lean landwards, and the back run would uplift it again with a jerk, leaving it with very little water about it, when the next incoming wave made it recline again. We did not credit the former inhabitants of the island, who remarked that the sea would reach the storehouse which we were building; and when these stones were said to have been moved it was treated with no credit, and was declared by all the workmen at the lighthouse works to be impossible ; yet the natives affirmed it to be so, and said if we were long here we might yet see it. They seemed to feel a kind of triumph when they called me to see it on the day of this great storm."

Thomas Stevenson, in The Mechanics Magazine, 1848
Thomas Stevenson was the father of the writer Robert Louis Stevenson and the son of Barra Head Lighthouse's builder, Robert Stevenson.