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PRISMATIC. 



In this division are included the common 

 prismatic, and, the more remarkable columnar 

 structures. 



The common prismatic structure is exempli- 

 fied in granite, and is rarely found except on the 

 large scale. It appears to be a modification of 

 the large lamellar structure ; a lamella being sab- 

 divided by joints, which, in this case, are at right 

 angles to it. Hence the forms are sometimes 

 nearly cubical ; in other .cases, they are quadrila- 

 teral prisms ; rarely presenting a greater or a less 

 number of sides. 



The prisms are frequently rounded at the an- 

 gles, so as to appear the consequences of a sphe- 

 roidal concretionary structure ; but this seems 

 almost always to be the effect of wear, or of par- 

 tial decomposition. 



In some cases, this wear proceeds gradually 

 from the circumference ; in others, the prisms 

 desquamate in crusts ; thus giving indications of 

 a lamellar structure united to the prismatic. For 

 the reasons already stated in treating of the lamell 

 lar structure, it must remain doubtful whether 



