250 J. D. Dana’s Mineralogical Contributions. 
from a figure in Mohs’s Mineralogy, pl. 13, fig. 97, and rendered 
nearly holohedral by adding (though of reduced size) the want- 
ing planes. In the occurring crystal, the right-hand J, 4, 2, 22 
and £2 are absent. Figure 2 represents an actual crystal of sim- 
pler form; it has but one plane J; and one plane 33 on either side 
is obsolete. 
Figures 3 and 4 represent known twin forms, copied with 
altered lettering from tracings received by the author from R. P. 
Greg, Jr. e plane 7 in these figures is made the base. 
The prism 47 has for its angle (at top) 120° 40’, which is very 
near the angle of a regular hexagon; so that these planes with . 
the planes 7 (fig. 1,2) make up a hexagonal prism, varying 20 to 
40 minutes from the angles of the regular hexagonal prism. 
And moreover as a consequence, the occurring planes between % 
and #7, and between @ and 43, are nearly alike in angle—J and 32 
inclining towards 7 at the same angle within 8’; and 7 (not 
shown in the figures,—situated between J and iz) and 4, at the 
same angle within 6’. Again, the macrodome 1i, like 4%, is neat 
120° in its angles, giving 119° 40’ and 60° 20/, the acute edge 
of the dome in this case being above. 
Brooke and Miller make the prism lettered 47 the fundamental 
prism, and 7 the basal plane. This gives simple expressions for 
the planes; but it does not appear to exhibit the true relations of 
the species. We arrive at this conclusion from the following 
considerations. 
The twins consist evidently of three united crystals, as 80 ' 
garded by Brooke and Miller, the plane 7 (or planes 10 the 
series 72, 77, ix’) forming three sides and only three out of the six 
(fig. 3). If the prism of 120° 40’ (47 above) be the fundamental 
prism, and analogous to that of Aragonite, the twins should be 
formed by composition parallel to the lateral planes of this prism, 
to either of them, and hence there is no similarity to any known 
twin in the Aragonite group. ‘This is seen 5 
in the annexed figure (fig. 5);—the sides of 
the hexagon lettered 47 are M of Brooke and iH ae 
Miller. c 
The composition is in fact parallel not to 44 
i (of figs. 1,2), that is M of Brooke and | 
iller, but to 1% which also is near 1209, #4 
having, as stated above, the angle 119° 40’. 
Hing 2 this psaray ot of Brooke and Miller) 
is better entitled from analogy and general aj 
principles, to be considered ee bala cnsel prism of Leadhillit 
than 4%. In fact the prism 12 (119° 40’) since it 1s 

