J. D. Dana’s Mineralogical Contributions. 83 
parent difference. The crystals are coated with Green earth, and 
uncovered. Qn investigating further these and other crystais, the 
writer finds that the crystals are in fact scalenohedral, although 
nearly identical with the rhombohedron (R), the obtuse term- 
inal edge (edge Y ) differing but ta from the inclined diagonal of 
. Its faces are smooth and polished. While the positions of the 
planes show that the form is dicially rhombohedral and not mon- 
oclinic, they also explain how different angles might be obtained 
in different directions, especially as the alternate pyrainidal faces 
of the scalenohedron are often very unequal. ‘This scalenohe- 
dron, since it is formed from a bevelment of the terminal edges 
of R, has the general sign ;--,R”. Theangle Y, (or that at the 
more obtuse or longer REL, edge,) by measurement is 175°— 
Li6°:; X. (or that at the acuter ot shorter edge,) is 979-989. 
These angles give approximately n =',°, and lead to the sign $R ‘. 
but calculation from this expression and the fundamental form of 
Chabazite makes the angle X = 100° 30’, which is too large ; for Y 
it gives 176° 8’, and as Z 84° 467. The measurements are how- 
ever only approximative. The same scalenohedron, apparently, 
occurs in the habeas crystals of Nova Scotia, but the planes 
in those crystals are so made up of strie that the angles are hardly 
measurable ; such striz have been considered an oscillation be- 
tween the planes Rand-sR. There can be little doubt that the 
Species is ee. and physically chabazite. he 
pce he are adverse to this conclusion. But Prof. Silliman states 
at he has reason s aoa his results; and in those by Delesse, 
fan of the iron was probably protoxyd, as obtained by Silliman. 
Small crystals of Heulandite (Beaumontite) cover the specimens, 
and are often implanted on the Chabazite. 'The Heulandite and 
green earth are probably of cotemporaneous formation, and sub- 
sequent to the Chabazite in origin. Some of the Chabazite crys- 
tals consists wholly of Green earth. 
5. Crystallization of Brucite (Mg 1). 
I am indebted to Mr. George J. Brush for a specimen of Bru- 
cite affording some minute crystals. amey es at druse 
upon the foliated brucite in a narrow 
sure in the serpentine. ‘The crystals prove Sere 
to be rhombohedral, as in the figure. The 
cleavage is basal, and the basal plane O is oes. 
pearly, ‘The other faces are brilliant vit- 
reous in lustre. ‘The crystals are so a ) 
nute that the inclinations could be 
ured with the reflective goniometer only be means 5 Teflected ss 
* 
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