CONSTRUCTION OF MINERALS. 



21 



cube and the octahedron can be converted into each oth- 

 er by cleavage. If the cube have its angles cleaved as 

 indicated by the dotted lines in Fig. 12, you have as a 





Jblg. 12. 



Fig. 13. 



Fig. 14 



result the body represented by Fig. 13. If you continue 

 the process, you will at length obtain the solid represent- 

 ed in the middle of Fig. 14 that is, the octahedron. If, 

 on the other hand, you take an octahe- 

 dron, and cleave it as represented in Fig. 

 1 5, you will at length obtain the cube as 

 indicated. These processes, and other 

 similar ones in regard to other forms, 

 may be gone through with very satisfac- 

 torily with raw potatoes and a common 

 knife. 



26. Secondary Forms. If with a knife you make such 

 cleavages as are represented in Fig. 8, you produce a 

 secondary form. Now, in nature, this result is not pro- 

 duced by cleavage, but by an omission to fill out the 

 whole figure, the omission falling short in various de- 

 grees in different cases. The omission may occur on the 

 angles instead of the edges, as seen in Fig. 7. 



Instead of omission there may be addi- 

 tion, as represented in Fig. 16. Here, by 

 an addition in the shape of a low pyramid 

 to each side of a cube, the dodecahedron is 

 produced. The addition on one of the faces 

 of the cube is shaded to make it obvious. 

 The increase is by layer upon layer of particles, each 

 layer having a less number of particles than the pre- 

 ceding one. This is indicated, in a coarse way, by 



Fig. 16. 



