738 . The American Naturalist. [September, 
angle of alstonite is determined as 119° 9’, which is not in accord with 
the view that it is an isomorphous mixture of witherite and aragonite, 
since their corresponding angles are 117° 48’ and 116° 16’ respectively. 
The indices of refraction of witherite, alstonite and barytocalcite for 
sodium light were measured and compared with those af aragonite and 
calcite. In the following table, column III gives the mean between the 
values for aragonite and for witherite : 
I II Ill IV V VI 
Aragonite Witherite Mean Alstonite Herglocalsite Calcite 
1.5301 1.52 .48625 
oe 1.5295 1.525 
B ae 1.676 1.679 1 1.673 (?) i 34 
Y -6859 1.677 1.681 1.686 1.6585 
Sp. G. 2.94 4.28 3.61 3.71 3.65 2.73 
Attention is called to the remarkable crystallographic similarity be- 
tween barytocalcite and calcite, nothwithstanding the difference in erys- 
tal system. The cleavage of barytocalcite form a pseudorhombohedron, 
being basal and prismatic. The angle of the prism 106° 54’, and the 
angle between the base and prism is 102° 54’, while the cleavage rhom- 
bohedron of calcite has angle of 105° 5’. Moreover, the optical angle 
of barytocalcite is small, and the negative acute bisectrix make an an- 
gle of + 64° 22’ with the c axis (i. e., with the intersection of the pris- 
matic cleavages) ; the optical angle of calcite is zero, and the negative 
optical axis makes an angle of +63° 44’ with the intersection of two 
rhombohedral cleavages. 
In conclusion Buchrucker’s values for the indices of strontianite are 
corrected. Mallard’s values for Na light are: œ = 1.518, 6 = 1.664, 
= 1.665. 
Rutile, Cassiterite and Zircon.—According to Traube, who 
discusses the question of the isomorphism of the above minerals, the 
etched figures produced by KF or KF HF are exactly similar for each 
of the three species, and indicate holohedral symmetry in the tetragonal 
system. 
An attempt to make rutile containing SiO, was not successful, 
though Traube considers that it must have been so in case rutile and 
zircon were isomorphous. Synthetic experiments were also made for 
the purpose of throwing light on the mode of occurrence of Fe,O, in 
these minerals. By heating chemically pure amorphous titanium diox- 
ide in a platinum crucible with sodium tungstate and a metallic oxide, 
the following results were obtained. With Fe,O, rutile was formed 
containing in one case as much as 5.4 per cent of that oxide; rutile 
‘Neues Jahrb. B.B. X, pp. 470-476, 1896. 
