303 SCIENTIFIC NEWS. 



posed western surface, produces this effect very sensibly. Irs 

 weight, as estimated from its dimensions, and the specific gra- 

 vity of granite, appears to exceed 65 tons. 



The subject of the second drawing is the Cheese-wring, 

 which occupies the highest ridge of a hill to the north of Lis- 

 keard. It is an irregular column, about fifteen feet high, com- 

 posed of five stones, the upper ones of which are so much larger 

 than the rest as to overhang the base on all sides. The angles 

 and external borders of these stones are considerably rounded 

 by the effect of decomposition j and there is no doubt that in 

 process of time, this disintegration will proceed so far, that 

 the balance of the pile will be destroyed, and its ruins will 

 not be distinguishable from the other bovvldens, with which the 

 tops of all the hills in this vicinity are overspread. 



The third drawing represents the Vixen Tor on Dartmoor. 

 Almost ail the granite of Cornwall and Devon, like that of the 

 Land's-end, is divided by fissures into masses more or less ap- 

 proaching a cubical form. If a rock of this kind, nearly level 

 with the surface of the soil, is examined, the fissures will be 

 found to be a mere mathematical plane, and the angles formed 

 by their intersection will be sharp and perfect. 



If we then turn our attention to granites, which, from their 

 greater elevation above the soil, appear to have been longer ex- 

 posed to air and weather, we may observe a gentle rounding of 

 the angles, such as is exhibited in the Vixen Tor. By degrees 

 the fissures become wider, and the blocks, which were origi- 

 nally prismatic, acquire an irregular curve-lined boundary, re- 

 sembling those which form the Cheese-wring. If the centre 

 of gravity chances to be high, and far removed from the per- 

 pendicular of its fulcrum, the stone falls from its support, and 

 becomes rounder by the progress of decomposition, till it as- 

 sumes one of the various spheroidal figures, which the granite 

 bouldens so often exhibit. 



These fissures, and the rounded form which the cubical 

 blocks acquire by decomposition, Dr. M. is inclined to attri- 

 bute to the original structure of the rock. In this, as in ba- 

 salt, crystallization appears to have been begun in distinct, and 

 more or less distant points, from each of which it proceeded, 

 forming thick concentric lamellae, till at length the exterior 

 shells of adjacent concretions came in contact,, but were inca- 

 pable 



