96 THE STRUCTURE OF THE [Ch. VI. 



This view of the subject is strengthened by the fact, that it 

 is not an uncommon arrangement in rocks to find not only 

 spheroidal masses, but the constituent parts so disposed 

 around common centres, as to form concentric layers. This 

 circumstance has been noticed in the gneiss of the Western 

 Isles of Scotland, and in the mica-slate of Norway ; but the 

 most remarkable instance occurs in the orbicular granite of 

 Corsica. 



The spheroidal structure is not confined to the component 

 concretions of the granitic rocks, but is characteristic of the 

 entire masses. Wherever the Cornish granite is exposed by 

 quarries on the side of hills, near the junction with slate (as 

 at Kitt Hill, and in the parishes of Mabe and Constantine), 

 the surface of this rock has not only a rounded form, but it is 

 also traversed superficially, by curved fissures or joints. 

 Indeed, the external form of the country gives indications of 

 this structure; for the broad rounded ridges constitute a 

 peculiar feature of the granitic districts of Cornwall and some 

 other countries, whilst the sharp spiry summits of the Alps 

 and^ of Arran distinctly mark a different structure, which is 

 confirmed by their being composed of tabular masses, resem- 

 bling strata placed on their edges. This tabular structure is 

 not only more prone to changes, on account of the com- 

 paratively greater exposure of surface, but also because it 

 offers greater facilities for the action of the elements. An 

 excellent example of this nature is presented by the granitic 

 elvans of Hanover Cove, which traverse the slate near the 

 granite of Cligga Point, in Cornwall: the softer kinds of 

 these rocks, and the intermediate slate, have decayed, and 

 been removed by the action of the waves ; while the harder 

 kinds of elvans project in thick tabular ridges, about two 

 hundred feet in height, the shattered and serrated forms of 

 which have a terrific aspect when viewed from the sea. But 

 to return to the spheroidal structure : there is one fact which 

 is opposed to its being the original or fundamental form of 

 granite, and that is, that the granite is not only traversed by 



