7° 7 NEW YORK STATE MUSEUM 
sions of the twinning habit are shown in figures 4 and 6; these as well as the 
twin crystals referred to above are figured as nearly as possible in their 
true proportions. 
Natural etch figures 
Plate 5, figures 2-5 
Natural etch figures, in some instances of sufficient size to be visible 
to the naked eye, were noted on the planes of 0 (0001), p. (1011), m. (4041) 
S$: (6281) and & (39.15.54.8) as follows: 
Basal pinacoid. The basal pinacoid as developed on a twin crystal of 
type I is covered with well defined etch pits of the form and arrangement 
shown in figure 2. The largest of these is 1 millimeter in length. The 
triangular depressions are bounded by the forms 6.(0112) forming the 
base of the isosceles triangle, and w:(3145) forming the sides. The acute 
angle measures 27° All three orientations of the triangular pits were 
noted on the basal pinacoid above mentioned. 
Rhombohedrons. The planes of the rhombohedron p.(1011) on the 
above crystal were deeply etched with triangular pits of the outline shown in 
figure 3. These were studied in detail on some of the smaller crystals of 
type II which offered a much smoother surface on which to observe them. 
The etch pits are oriented with their straight sides parallel to the rhombo- 
hedral edges and are bounded on the straight sides by planes of p. The 
curved side is formed by the penetration of a plane or series of planes of 
slightly steeper inclination than p., possibly by the planes of m. or n. 
The apparent lack of symmetry with respect to the short diagonal of the — 
rhomb disappears when the etch pits b and c are compared with d, it being 
highly probable that the two former outlines represent the occilatory 
influence of one scalenohedral plane. 
Minute etch pits of the form shown at e, figure 4, were noted ona 
well developed plane of the rhombohedron m. (4041) from a small crystal 
of type III. They are bounded by three figure planes which outline the 
isosceles triangle and a bottom plane. The vertical angle of the triangular 
outline measures 83°. The outline suggests the possible identity of the 
basal figure plane, bottom plane and side planes of the etch pit with the 
planes of p.(1011), m. (4041) and 9%: (6281) in the order named, 
