CALCITES OF NEW YORK 10g 
of 50 millimeters measured on the composition plane. The faces are dull 
and the crystals are black or dark gray in color as the result of bituminous 
inclusions. In general habit they are similar to those described under type 
II, the following forms being noted :-—M: (7.4.11.3), vy (8.8.16.3) and b (1010), 
the latter only occasionally present. 
Twin crystals of this type, according to three of the four laws known 
to calcite, are very common, viz: 
Parallel to the basal plane o (0001) 
Parallel to the rhombohedron 3. (0112) 
Parallel to the rhombohedron ¢. (0221) 
The latter of these laws is rare for calcite. Figure 1 shows an untwinned 
scalenohedral crystal on which the position of the three twinning planes is 
indicated. Figures 2, 3 and 4 show crystals of type V twinned according 
to the above laws.‘ The twin crystal habits shown in figures 3 and 4 are 
invariably distorted to the extension of one set of axially opposite scaleno- 
hedral planes; this distortion operates in the case of the crystals twinned 
parallel to ¢.[fig. 4] to the suppression of the reentrant angles ordinarily 
characteristic of twinning. 
Type VI [pl. 19, fig. 5,6]. This type is found in crystals of the sec- 
ond generation which occur deposited on a thin layer of first generation 
calcite of rhombohedral habit (type III). They differ from all which have 
been previously described in two essential characteristics: they are opaque 
and milky white in color and show a complete absence of all marcasite or 
pyrite inclusions. The crystals are scalenohedral in habit, having for 
dominant forms the scalenohedrons M: (7.4.11.3) and 9: (19.10.29.6). The 
latter form which is in the zone [4041.8.8.16.3] is also present on crystals 
of type III. The rhombohedrons p. (1011), m. (4041) and 8. (0112) and the 
‘In figures 3 and 4 the writer has followed the excellent method inaugurated by 
Messrs Penfield and Ford, of projecting the twin crystal with its twinning plane vertical 
and parallel to a hypothetical plane 1210 of the ordinary projection. 
