SYSTEMS OF ORYSTALLIZATION. 17 
species and crystalline forms that are very close in their rela- 
tions. There are forms under each of the systems that differ 
but httle in angles from some of other systems: for example, 
square prisms that vary but slightiy from the cubic form; tri- 
clinic that are almost identical with monoclinic forms; hexa- 
gonal 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 
erystallographic system is far from certain evidence that any 
species are fundamentally or widely unhke. 
I. THE ISOMETRIC SYSTEM. 
1. Descriptions of Forms. The following are figures of some 
of the forms of crystals under the isometric system : 
it Re 

a 
The first is the cube or hexuhedron, already described. Be- 
sides the three cubic axes, there are equal diagonals in two 
other directions ; one set connecting the apices of the diago- 
nally opposite solid angles, fowr in number (because the number 
of such angles is eight), and called the octahedrul axes ; and 
wnother set counecting the centres of the diagonally opposite 
2 
