648 



GEOLOGY. 



granite in the Alps that can be compared with it for the elegance of its form, or for the 

 length of its shaft. The Geant, it is true, is nearly equal to Mont Blanc in height, but 

 it does not rise so far above its base as the Aiguille de Dru." The view exhibits this 

 extraordinary shaft the Aiguille Vert behind it the Glacier de Bois descending into 

 the valley, a continuation of the Mer de Glace and the Arveiron flowing at the bottom. 

 Though several specimens of ancient art are composed of granite, and, as the monu- 

 ments of Egypt bear witness, are of extreme durability, having stood the wear and 

 tear of thousands of years without much injury, the existing state of granitic rocks 

 of the hardest texture significantly intimate the destructive effect of agencies acting upon 



them through longer intervals of time. A great number 

 of examples occur around the islands of Scotland, of huge 

 ^tlfi^-s^ masses of granite having been worn away, and separated 

 into fragments, by the action of the sea, as here repre- 

 sented : while to the north of Aberdeen, the bold granite 

 coast has been singularly hollowed out, by the play of 

 the waves, into arches and caves the Bullers of Buchan, 

 so called from the peculiar noise produced in them by 

 the stormy ocean ; and the summit of Mount Bennachie 

 supplies an example of the manner in which granite 

 yields to the decomposing power of the atmosphere, 



having been separated into huge masses or blocks, irregularly piled, by the mouldering 

 away of the base or paste. The felspar constituent is the first to decay, and that rapidly, 

 if it contains soda or potash in excess, crumbling into sand and dust, which the rains 

 carry down into the valleys, and accumulate in beds of clay, in which state the Chinese 

 employ it in the manufacture of their finest porcelain. The celebrated Logan stone 

 near the Land's End in Cornwall an enormous mass of granite, said to weigh more 

 than sixty tons which is so delicately poised on the top of other rocks, that a slight 

 force, the strength of one man, is sufficient to set it in motion is an instance of this 

 process. Tradition, indeed, regards it as a contrivance of the Druids, which they 

 employed as an engine of superstition a kind of ordeal. 



" Behold yon huge 



And unhewn sphere of living adamant, 

 Which, poised by magic, rests its central weight 

 On yonder pointed rock ; firm as it seems, 

 Such is its strange and virtuous property,. 

 It moves obsequious to the gentlest touch 

 Of him whose breast is pure ; but to a traitor, 

 Though e'en a giant's prowess nerved his arm, 

 It stands as fixed as Snowdon." 



The position of the stone is the result of the natural process of disintegration, by which 



granite rocks are often sepa- 

 rated into rhomboidal or 

 square masses, gradually as- 

 suming a spheroidal or 

 rounded form as the decay 

 proceeds. De Luc observed 

 in the mountains of Silesia 

 several of these spheroids, so 

 piled upon each other, that 

 he compared them to a 



Disintegrated Granite. number of Dutch cheCSCS, US 



