88 



STRUCTURE OF MINERALS. 



the terminal cannot be truncated, but are replaced by planes 

 unequally inclined to the including planes. The solid angles 



49 50 



48 



^^5^ 



P^ 



P 



of the square prism are of one kind and are replaced alike, as 

 in figures 23, 50 ; all the angles in these figures have the 

 same number of planes, and the two adjacent planes in figure 

 50 are similar in their inclinations, because the lateral planes 

 M, M, of a ..square prism, are equal. 



In the rectangular and rhombic prisms the lateral axes are 

 unequal. Consequently in the rectangular prism, two basal 

 edges differ from the other two, and are therefore modi- 

 fied independently (figs. 51, 52.) The planes e extended to 

 the obliteration of T and P, would produce a rhombic prism 

 (in a horizontal position,) as shown in figure 53, and another 

 horizontal prism may be formed by the extension of the 

 planes e, fig. 52. In the rhombic prism the basal edges coi 

 51 52 53 54 



f^\ p .-A 



>-■#-•- 



T 



! M 



\->~f 



w 



55 respond to the angles of the rectangular prism 

 <TjT^> 1 (see fig. 265) and are similar and simultaneously 

 replaced as in figure 24. The basal angles are 

 unlike, one being obtuse and the other acute, and 

 the planes of the two (fig. 54) differ in their in- 

 clinations. The laterai edges differ in the same 

 manner, two being obtuse and two acute, and they are inde- 

 pendently replaced, as in figure 55. The two planes e are 

 similar planes, because, in a rhombic prism, M and M are 

 equal ; and the extension of these planes may produce another 

 rhombic prism. 



In an oblique rhombic prism the superior basal edges dif- 



Explain these laws from the square prism; the rectangular and 

 rhombic. 



