216 J. D. Dana’s Mineralogical Contributions. 
Datholite, as Brooke and Miller show, is trimetric and hemihe- _ 
dral, instead of monoclinic, its hemihedral character giving ita 
monoclinic aspect. Still it is hom@omorphous with Euclase. 
In Euclase, I: I=114° 50’; and a: b : c=0-4894: 1: 1-477. 
In Datholite, l: 1=115° 26-12’); a: b: e=0-5: 1: 1:6829. 
In each, the axes are very nearly as 1: 2: 3. 
The homeomorphism of Sphene and Euclase is shown by the 
author in this Journal, vol. xvi, [2], p. 96. 
14. Isodimorphism of Tourmaline and Calcite. 
In the last number of this Journal, the author has written for 
the formula of Tourmaline, (R*,#, 8): Si*, (which is equivalent 
to (R*, #, B) Siz.) ie ot da 
The species Euclase is known to have the formula (41, Be)! Si. 
The analogy in these formulas of Tourmaline and Euclase 
will be observed. And if an analogy in crystallization existed, 
we should have thereby good evidence that the formula of Tour- 
maline was right in fact and also in principle. / 
Tourmaline and Calcite are closely homceomorphous, as was 
sometime since suggested to the writer by Mr. T. S. Hunt. fed 
rhombohedron 2R of Tourmaline has the angle 103°, near 105 °5 
of Calcite. The planes 2R are as highly polished as R, and 
" sometimes more highly so; and there is no reason in cleavage oF 
called R (giving the angle 133°8) / 
become $R, analogous to the com- 
mon nail-head form in Calcite. No 
objection therefore exists to the ho- 
mcomorphism of these species. The 
annexed figure presents a new form 
of Tourmaline, from Hunterstown, 
Canada East, for the privilege of study- : 
ing which the writer is indebted to 
Mr. Hunt. It is of arich dark-brown color and transparent, nA 
uring two-thirds of an inch across. It is lettered to correspoD 
with the above views, the plane 6. Me 
usually called 2R being made R. 
Calcite and Barytocalcite are i 
well known to be mutual di- 
=) 
pee 
= 
