GRANITE. 95 



until, at length, the masses become separated, as if 

 they had never been connected. Thus they acquire 

 the appearance of piles of masonry, often putting on 

 strange forms, like the Cheese-wring in Cornwall, and 

 producing the well-known rocking stones. Under a 

 further decomposition at the angles, these masses be- 

 come a heap of irregular spheroids, which, being often 

 easily removed, roll down the declivities of the hills, 

 forming some of the travelled blocks so often discussed. 



Among these indications of a peculiar internal struc- 

 ture, I have formerly described the exfoliations into 

 spheroidal crusts and schistose Jaminae, so as to render 

 any further notice of them unnecessary. Decided 

 marks of concretionary arrangement are also found in 

 the occasional orbicular tendency of the component 

 parts ; familiar in the well known variety of Corsica, 

 and in " Tiger granite," and sometimes marked in a 

 circular distribution, affected by the mica in ordinary 

 granite. They all present analogies to the similar 

 structures occurring in such porphyries as that of Cor- 

 sica, in the Traps, and in the products of earths fused 

 by artificial heat ; and I need not now repeat their 

 bearing on the igneous origin of these rocks. 



The last circumstance in the geological character of 

 granite, relates to its distribution in the form of veins ; 

 of which there are two distinct kinds. The first lie 

 wholly within the rock, consisting of the same mate- 

 rials, under slight differences in the colour and mag- 

 nitude of the parts, being also connected with similar 

 variations, of a concretionary appearance, without the 

 venous form. If the exact cause of these is not 

 obvious, they are connected with that venous struc- 

 ture formerly described, which is only detected by the 

 exposure or disintegration of the rock. 



The next are much more interesting, and constitute 



