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fcontal ; and, in these cases, it is implied that the 

 prisms are straight, as well as parallel to each 

 other. 



The parallel position is not however necessary. 

 In a collection of prisms, they are sometimes 

 found to radiate from some imaginary centre, or 

 from more than one^ In other cases, they are 

 placed in various irregular ways and entangled 

 together. In some instances, such irregular 

 prisms are found more or less accumulated, or 

 dispersed, arid intermixed with the same rock in 

 an amorphous state. 



When a collection of prisms is parallel and 

 erect, with straight axes, and on a large scale, the 

 well known effect of architectural regularity is 

 produced. 



The prisms are not necessarily straight. At 

 times they are slightly, at others, very conspicu- 

 ously curved. Sometimes the same curvature 

 affects a considerable associated number ; or the 

 prisms are then bent in a manner more or less 

 parallel. In such examples, the general effect of 

 regularity is not destroyed. In other instances, 



K 2 



