ad 
252 J. D. Dana’s Mineralogical Contributions. 
he rhombohedral and monoclinic forms, and the trimetric and 
rhombohedral, have nearly a common difference, 10 to 11 degrees; 
which differs by some similar angle from Susannite. The paral- 
lelism above is so exact both vertically and horizontally, that the 
argument must be allowed to have much weight. Its authority 
becomes irresistible when viewed in a different light. 
3. Dreelite is dimorphous with Anglesite and Barytes,—the 
same compound, essentially, occurring here under a rhombohedral 
and trimetric form. Moreover, Dreelite and Susannite are identical 
sulphato-carbonate under any other form it may present, of 10 
Leadhillite; whence in either case, the forms should be home@e 
morphous with the corresponding sulphates. 
t seems therefore to follow that 103° 16/ is the true vertical 
this case, the most perfect cleavage would be basal, which !s not 
true of any known species of the aragonite group. But after the 
arguments above stated, we hardly need look further for evidence. 
An objection to the view adopted might be suggested from the 
planes [is 
ould not 
and the 
from Brooke and Miller that the plane J is also a direction © 
twin composition; shows that this plane is at least one of promt 
nent or fundamental value in the crystal. : is 
Upon the view which has been discussed, the species Ang" 
site, Anhydrite and Leadhillite have the following dimensions - 
Pisin Th eli Dome 1 Axes a: b =f 
Anglesite 103° 38’ ss 62° 49" 75° 297 16415: 1: eit 
Anhydrite 102° 56’ 61° 25/ "9° 38’ 16886: 1: Pee 
Leadhilli lite 103° 167 60° 20° 49° 34’ 17205 :1:1 
