ROCKING-STONES 449 



ARTIFICIAL ROCKING-STONES AN EXPERIMENT. 



Communicated to the Geological Sec. of the British Association, Norwich. 

 Norfolk Mws, Aug. 25, 1868. 



SOME short time ago, during an excursion in Cornwall, my 

 attention was naturally directed to rocking-stones, and those 

 approximations to rocking-stones which are seen in the granite 

 where it is exposed to the action of the heat and cold, air and 

 water. I presume that I need not argue here that rocking- 

 stones are natural results, and not superposed on their pedes- 

 tals, as was once believed, by the hand of man. 



Throughout the greater part of the granite rocks of the 

 west coast of Cornwall formations are to be seen approaching 

 in character to rocking-stones, or to discoid piles, like the 

 Cheesewring. 



If we suppose a slab of stone of a parallelepiped form 

 lying on another, both having flat surfaces, or in other words 

 such slabs as are formed by fissures in horizontal and perpen- 

 dicular directions which are common in exposed granite rocks, 

 the attrition and disintegration produced by changes of wea- 

 ther, of temperature, &c., would necessarily act to the greatest 

 extent at the corners and next to that at the edges, because 

 those parts expose respectively the greater surfaces compared 

 with the bulk of the stone. This would tend to round off all 

 the angles, and gradually change the rhomb more or less towards 

 an oblate spheroid. This would account for the Cheesewring, 

 &c. But then, it may be asked, why should this process gra- 

 dually work on to a rocking-stone ? in other words, why should 

 the last unworn point, points, or line be in the line joining the 

 centre of gravity of the upper stone with that of the earth ? 

 Such an accident, it may be said, might happen, but the 

 chances are almost infinity to a unit against it. Not so. 

 Assume the wearing away between the slabs to reach a point 

 which is not in the line of centres of gravity, the upper stone 

 would then fall on one side, leaving the unworn point more 

 exposed to climatal and probably to electro-chemical action 

 from the water lying in the angle of the crevice, evaporation 



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