133 



exclusively under that head ; but, to have omitted 

 them in the present chapter, would have left this 

 account of the forms and structures of rocks im- 



x 



perfect, 



On the small scale, as in columnar ironstone, 

 which is but a modification of shale, a collection 

 of prisms is not only curved but sometimes mi- 

 nutely undulated. 



The prismatic structure is not limited to beds, 

 or masses, but occurs also in veins. In this case 

 the prisms are most commonly at right angles to 

 the plane of the vein. But they have been also 

 found to occur in a direction parallel to it, and 

 either horizontally or vertically placed. 



The columnar structure is sometimes com- 

 bined with the lamellar ; and the lamellae may 

 either be parallel, or at right angles, to the axis 

 of the prism. 



It is almost unnecessary to remark, that the 

 prismatic does not exclude the minute kinds of 

 structure, namely^ the amygdaloidal and the 

 porphyritic. 



In an aggregate of prisms, they are always 

 found in contact, except in the case of an union 



