88 



MINEEALOGT. 



A rhomb, fig. 13, has four equal sides, but its adjacent 

 angles, a and &, unequal. 



FIG. 12. 



FIG. 13. 



A rhomboid differs from a rhomb, in having only two, 

 instead of four, equal sides, as fig. 14. 



z 



FIG. 14. 



FIG. 15. 



An oblique-angled parallelogram, fig. 15, has its opposite 

 sides parallel, but its adjacent sides, a and b, and its adjacent 

 angles, c and d, unequal. 



When certain forms of crystals are described with refer- 

 ence to the rhomb, as the figure of their planes, they are 

 termed rhombic. 



A parallelepiped is any solid contained within three pairs 

 of parallel planes. 



Crystals often present the appearance of having lost then- 

 edges and solid angles, which are then said to be replaced 

 by tangent planes. A tangent plane, with reference to an 

 edge, signifies a plane inclined equally to the two adjacent 

 primary planes, and parallel to the edge which it replaces ; 

 or, in plainer terms, it means cutting off the edge so evenly 

 as to take off" as much on one side as the other, as in the cube, 



or octohedron, 



FIG. 16. 



FIG. 17 



and the same description applies to a solid angle. 



When an angle is replaced by two such converging planes, 



