Thither, youths,
Turn your astonish'd eyes; behold yon huge
And unhewn sphere of living adamant,
Which, poised by magic, rests its central weight
On yonder pointed rock: firm as it seems,
Such is the strange and virtuous property,
It moves obsequious to the gentlest touch
Of him whose breast is pure; but to a traitor,
Tho’ ev’n a giant’s prowess nerv’d his arm,
It stands as fixt as Snowdon.

http://en.wikipedia.org/wiki/Rocking_stone

So says the poet on Wiki about a logan stone; a traitor would be found out if the stone moved in the wrong direction, at least so the folklore says about the 'druidic' powers of a logan stone ;)

http://www.themodernantiquarian.com/site/1719/logan_stone.html