GUIDE TO THE MINERALOGIC COLLECTIONS 



7 



the mineral zircon, both being composed of the faces 

 marked p and m. An examination of fig. 5, 6, 8 and 9, 



Fig. 4 



Fig. 5 



Fig. 7 Fig. 8 Fig. 9 



which show sections through fig. 4 and 7, will demon- 

 strate that the angles between m and m in fig. 5 and 

 8 are equal as are also the angles be- 

 tween p and p and between p and m in fig. 

 6 and 9. Compare the cardboard model 6 1 , 

 which shows the same crystal as fig. 4. Fig. 

 10 shows a distorted octahedron of magnetite, 

 the shaded form within the outline represent- 

 ing the normal octahedron equally developed 

 in all directions; the faces of the outer and inner forms are 

 parallel each to each. The cardboard model 1 is an ideal 

 octahedron. 



Law of $ijmmetrij 



By symmetry is meant the degree of regularity with which 

 the faces and angles of a crystal are grouped about points, lines 

 and planes. Thus a crystal may be symmetric to a plane, sym- 

 metric to a line or axis, symmetric to a point or center. 



Fig. 10 



The cardboard models will be found in a pocket attached to the cover. 



