80 THE CONSTITUTION OF THE SILICATES 
which compares with Rammelsberg's analysis as follows: 
[BULL. 125. 
Found. Calculated. 
I 
Ti0 2 
21.20 
17.55 
32.51 
19.41 
3.10 
3.34 
2.50 
20.70 
18.63 
31.52 
) 23. 14 
3.38 
2.63 
Tli0 2 
Cb 3 5 
(Ce, La, Di) 2 3 
(Y,Er) 2 3 
FeO 
Ca 
99.61 
100. 00 
For polymignite a similar expression gives equally good results. It is 
5 (Cb0 3 )2=Zr=Ti0 4 =(Fe /// 0) 2 
4 0=Zr=Ti0 4 =(CeO) 2 
15 O=Zr=TiO 4 =0a 
1 0=Th=TiO,=Na» 
The similarity in type of these expressions to the orthosilicate for- 
mula for sphene is obvious. It is also obvious that the entire group is 
greatly in need of further study. 
