The New Student's Reference Work/Rocking-Stones

Rock′ing-Stones or Logans are large masses of rock so finely poised as to move backward and forward with the slightest impulse. Some rocking-stones occur near remains of ancient fortifications, which seems to confirm the statement, in one of the poems of Ossian, that the bards walked, as they sang, round the stone and made it move as an oracle of the fate of battle. In Greece rocking-stones occur as funeral monuments, and are generally found on conspicuous places near the sea. The rocking-stone of Tandil in the Argentine Republic, 250 miles south of Buenos Aires, weighs 700 tons, and yet is so nicely poised that it rocks in the wind and may be made to crack a walnut.