Rocking Stones, or Logans, large masses of rock so finely poised as to move backward and forward with the slightest impulse. They occur in nearly every country. Some of them appear to be natural; others artificial; the latter seem to have been formed by cutting away a mass of rock round the center-point of its base. The former are chiefly granitic rocks, in which felspar is abundantly present. Various explanations have been given of the uses of these singular objects. They are supposed to have been used in very early times for purposes of divination, the number of vibrations determining the oracle; hence it came to be believed that sanctity was acquired by walking round them.
The famous rocking stone of Tandil in the Argentine Republic, 250 miles S. of Buenos Ayres, weighs over 700 tons, yet is so nicely poised that it rocks in the wind and may be made to crack a walnut.
Entry from Everybody's Cyclopedia,