476 OVERLYING ROCKS. 



separable by weathering, as they are in the sili- 

 ceous schists of this character. 



Besides these obvious modifications, the rocks 

 of this family sometimes give indications of a 

 particular internal structure, by decomposition. 

 Thus they put on a scoriform or a spicuiar appear- 

 ance, or are proved to contain veins of a harder 

 material, which are not visible in the fresh rock. 

 They frequently also decompose by a successive 

 exfoliation into spheroidal forms, of which no 

 indications were present ; and it is not uncommon 

 for the portions of jointed columns, to waste in 

 this manner. In this latter case, it may remain 

 a doubt whether, as in the artificial columns of 

 granite formerly mentioned, the etfect is not pro- 

 duced merely by the action of the weather ; but 

 in some, where this cause cannot have acted in 

 this manner, it must necessarily be the result of 

 an internal concretionary arrangement. 



On the subject of decomposition, it is lastly 

 proper to remark, that some of the more solid 

 varieties, such as those which have a base of in- 

 durated claystone, or clinkstone, are sometimes 

 changed, without disintegration, to so great a 



