CALCITES OF NEW YORK 33 
Zonal relations. Examination of a suite of crystals of any crystallized 
mineral will render apparent the fact that certain planes in combination 
are so related that the series of their intersecting edges are parallel. Sucha 
series of faces are said to lie in a zone.' Thus in figure 18 the edges B and D 
are parallel in three series, corresponding to the three Miller axes which in 
this case are zone axes, and consequently the faces of the unit rhombohedron 
lie in three zones each composed of four faces. The crystal of calcite 
illustrated in figure 19, the faces of which are designated by the Bravais 
symbols, shows three well developed series of zones, viz: 
1 The series having its edges parallel to the 
rhombohedron R (1011) and including the faces 
AS Ole sta St. Lit Olete: 
2 The series having its edges parallel to 
— 2R (0221) [see fig. 10] and including the faces 
0221, 2461, 1120, 4261, 2027 etc. 
3 The series having its edges parallel to the 
short polar edges of R3 (2131) [see fig. 16] and 
meladme 21310221 230 etc: 
-Each of these zones indicated is, by reason of the fundamental 
symmetry of the crystallographic group to which calcite belongs, repeated 
three times. Thus the zone [1011.1120] corresponds to [1011.2110] and 
to [0111.1210]. 
Any zone is determined by the direction of the intersecting edges of its 
planes and consequently by the direction with respect to the crystallo- 
graphic axes of a line parallel to these edges and passing through the center 
of the erystal. Such a line is called the zone axis. The indexes which fix 
the position of the zone axis may be derived from the Bravais symbols of 
two planes in its zone by the following method: 
1 The conception of a crystal encircled by a zone may be facilitated by the considera- 
tion of figure 5 which shows the fundamental rhombohedron intersected by the planes 
of the prism of the second order. The six planes of the latter form lie in a vertical 
zone. 
