32 



STRUCTURE OF MINERALS. 



28a. Looking down again on the model as before, the lat 

 eral angles are seen to form six equi-distant points around tnt 

 axis ; and if these angles are removed in the same manner, 

 another six-sided prism is obtained, differing, however, from 

 the former in having the faces of the pyramid at each end, 

 five-sided, instead of rhombic. Figures 29, 30, illustrate the 

 process. Conversely, we may make a rhombohedron out of 

 28 28a 29 30 31 



^^ 



VJ 



a hexagonal prism, by cutting off three alternate basai edges 

 at one extremity of the prism, and similarly, three at the 

 other extremity alternate with these, as in figure 31. In fig- 

 ure 30, the process is farther continued, and the rombohedron 

 is shown as a nucleus to the prism. By cutting orf slices 

 parallel with R, the rhombohedron is at last obtained. The 

 close relation of the rhombohedron and hexagonal prism is 

 hence obvious. Calcareous spar has the rhombohedron as its 

 primary, and very often occurs in hexagonal forms. The 

 same is true of quartz and many other species. 



From the above transformations, the study of which, with 

 the aid of a knife and a few raw potatoes or lumps of chalk, 

 may afford some amusement as well as instruction, the stu- 

 dent will understand more fully the six systems of crystalli- 

 zation.* r These six systems have received the following 

 names : 



1. Monometric or tesseral system, (from the Greek monos, 

 one, and metron, measure, alluding to the three axes being 

 equal in length.) Includes the cube, octahedron and dode- 

 cahedron, (figs. 1, 2, 3.) 



2. Dimetric system, (from dis, two times, and metron, al- 

 luding to the vertical axis being unequal to the other two.) 



Give the names of the systems of crystallization, and mention the 

 forms each includes. 



* In some text books, the student may read about certain integral 

 f orms, the cube, the three-sided pyramid and tlree-sided prism, from 

 *hich it is stated all the other forms may be made. The idea of sw.h 

 forms has nothing to do with crystallography, jr the actual conutitu 

 Gon of crystals. 



