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the curvatures are variously opposed ; or the bent 

 are intermingled with straight prisms. 



A columnar mass is sometimes formed of a 

 collection of short prisms, placed in a manner 

 more or less parallel, but so that the extremities 

 of some are in contact with the middle parts of 

 others. In such cases they impress each other ; 

 and the effect of regularity in the structure of the 

 mass, nearly disappears. A remarkable example 

 of this occurs in the Scuir of Egg. 



As the prismatic is united to the bedded, or 

 pseudo-stratified form, so it occurs together with 

 the spheroidal concretionary ; but, in the only in- 

 stances yet known, the prisms are short and 

 united with the amorphous mass. This circum- 

 stance was noticed in treating of the spheroidal 

 structure, and is found in the sandstone of Egg. 

 It also occurs in Rum, in trap ; the spheroid 

 being of an enormous magnitude. 



The preceding remarks on aggregated prisms, 

 like many of the following, apply chiefly to the 

 rocks of the overlying family, trap, syenite, and 

 porphyry. They might have been mentioned 



