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various. They are sometimes nearly or abso- 

 lutely spherical ; in which case they touch by 

 points only, and the intervals are filled by the 

 same, or by another substance. Examples of 

 this are to be found in some siliceous schists and 

 in some limestones. 



In other cases, by compressing each other, 

 they become oblate, or indented, or assume va- 

 rious irregular shapes : and they thus sometimes 

 form a mass which presents an aspect as much 

 granular as it is concretionary. Some of the 

 shales which are found in contact with trap rocks, 

 present examples of this peculiar structure, and 

 they are found, very conspicuously, in the Shiant 

 isles. 



Such concretions further possess at times a 

 distinct lamellar, or a radiated structure ; and, 

 in one rock (pearlstone), they sometimes con- 

 tain a central particle of another substance. 



Rocks of this structure often acquire a bo- 

 tryoidal surface after exposure to the weather ; 

 and, in some instances, this concretionary ar- 

 rangement is so concealed in the apparently uni-? 

 form fracture of the rock, that it is only distin- 



