E. §. Dana—Chondrodite from the Tilly-Foster Iron Mine. 95 
rarely was a variation found greater than two minutes. The 
preceding table contains the supplement angles for each of these 
minute planes as measured on Cand r?, and also on 7?. 
The calculated symbols are also given, with the angles which 
belong to them. It will be noticed that 7? is itself one of the 
minute planes, of the same character as those surrounding it; 
and its presence gives a reality to them which they would not 
otherwise have, and shows what degree of reliance is to be 
placed on the angles. 
impossible: thus, in type II, 1:4: 4:4; andin type III, 1, 4, 3,3, 
4, ts ti an 
should themselves have symbols totally at variance with the 
accepted law of simplicity of the indices. 
Tt will be noticed, however, that, lawless as they appear at 
first, there is an attempt at system in the symbols given. 
Sta in the ratio of the brachydiagonal to the vertical axis, we 
ave: 
aren © tie y® 24: 18 ot ee a ae 7 
ee 18: 4 gt 24s "6 gi?" [gr 7 22 26: 9 
y® 24: 1 ws 13: 12 
and so on 
Almost all of the twenty and more smaller crystals exam- 
In the symbols given in the tables (i. e., those according to Naumann) the 
mark over the second figure, or has been omitted (in order to simplify the 
work of the : . This has also been done in all the following being 
fa a Possible by the fact that all these planes, with one or two exceptions, belong 
