CALCITES OF NEW YORK sat 
1 Twinning parallel to the basal plane is very common, the vertical 
axis of both elements lie in the same line. Figure 21 shows the scaleno- 
hedron K: (2131) twinned according to this law. 
2 Twinning parallel to the negative rhombohedron 8. (0112) is common; 
the vertical or polar axes of the two elements of the composite crystal form 
an angle of 127° 293’. Figure 22 shows the positive scalenohedron K: (2131) 
twinned according to this law. 
3 Twinning parallel to the positive rhombohedron p. (1011) is uncom- 
mon; the vertical or polar axes of the two elements of the composite crystal 
form an angle of 90° 46’. Figure 23 shows the positive scalenohedron K: 
(2131) twinned according to this law. 
Fig. 22 Fig. 23 Fig. 24 
4 Twinning parallel to the negative rhombohedron ¢. (0221) is rare; 
the vertical or polar axes of the two elements of the composite crystal form 
an angle of 53° 46’. Figure 24 shows the positive scalenohedron K: (2131) 
twinned according to this law. 
The position of the twinning plane in the first, second and fourth of the 
above cases is shown in plate 19, figure 1. 
Formulas for calculating angles. For the identification of crystal 
forms of calcite it is necessary to compare the interfacial or the coordinate 
angles of the observed form, as measured by some form of goniometer, with 
the corresponding theoretical angles as calculated from the assumed indexes 
of the form. All types of reflection goniometers read the normal angles 
