SYSTEMS OF CRYSTALLIZATION. 



17 



species and crystalline forms that are very close in their rela- 

 tions. There are forms under each of the systems that differ 

 but little in angles from some of other systems : for example, 

 square prisms that vary but slightly from the cubic form ; tri- 

 clinic that are almost identical with monoclinic forms ; hexa- 

 gona? that are nearly cubic. Consequently it is found that the 

 same natural group of minerals may include both trimetric and 

 monoclinic species, as is true of the Hornblende group ; or 

 monoclinic and triclinic, as is the fact with the Feldspar group, 

 and so on. It is hence a point to be remembered, when the 

 affinities of species are under consideration, that difference in 

 crystaUographic system is far from certain evidence that any 

 species are fundamentally or widely unlike. 



L THE ISOMETRIC SYSTEM. 



1. Descriptions of Forms. The following are figures of some 

 of the forms of crystals under the isometric system : 





7. 



The first is the cube or hexahedron, a^eady described. Be- 

 <rides the three cubic axes, there are equal diagonals in two 

 other directions ; one set connecting the apices of the diago- 

 nally opposite solid angles, four in number (because the number 

 of such angles is eight), and called the octahedral axes / and 

 another set connecting the centres of the diagonally opposite 

 2 



