The ‘Alternative Approaches to Folklore’ bibliography at
hoap.co.uk/aatf1.doc
mentions that the stones at Ballochroy were thrown by Brownies (one assumes the little people type, not the bobble-hatted sort). Do you know more about the story? Tiny Cara Island, just across the Sound of Gigha, has its ‘Brownies’ Chair’. Perhaps they threw them from there (though you wouldn’t normally think them so strong).