110 EXERCISES ON THE 



replace the slices on the octohedron, till you restore it to a 

 cube (figs. 21 and 48). 



11. Form an octohedron for the like purpose, reduce it 

 to a cube, and, by replacing the slices, restore it to an octo- 

 .hedron (fig. 33). 



12. Commit to memory the names of the fifteen primary 

 forms, page 92, et seq. ; study their characteristic differences, 

 as well as their geometrical relations ; copy their outlines on 

 tracing paper, and study them by the assistance of a series 

 of models. 



13. Describe their appropriation in the system of Mohs. 



EXEECISES 



ON THE GENERAL CHARACTERS OE MINERALS. 



1. WE would here repeat the recommendation to commence 

 reading with much care the Introduction to Phillips' s Minera- 

 logy, and to devote peculiar attention to the chapter on 

 structure, as well as to the various sections which treat of 

 external form, fracture, hardness, double refraction, and 

 specific gravity. 



2. Commit to memory the divisions of mineral substances, 

 as enumerated by Phillips and Griffin, and learn the chief 

 types of each class. 



3. There are some half-dozen different substances which, 

 from their occasional resemblance to each other, should be 

 studied with peculiar assiduity; and the pupil should ac- 

 custom himself, both in theory and practice, to distinguish 

 between them, both as single minerals, and in their com- 

 bination in rocks. These are quartz, carbonate of lime, 

 hornstone, feldspar, augite, and hornblende. With a view to 

 this object 



4. Describe quartz under its general physical characters 

 of mineral composition, hardness, fusibility, forms and loca- 

 lities of occurrence, crystalline form, and different species 

 and varieties 



