E. /S. Dana — Ghondrodite from the Tilly-Foster Iron Mine. 85 

 Table XI. 



Besides the interest of the twin, a noteworthy fact in the crystal is 

 the occurrence of the prism (/-2=210) the first time that any one of 

 the vertical prisms has been ohserved in the 2d tyj^e of either humite 

 or chondrodite. Tt lies in botli the zones e-(201), /^^ (2^1), and e2(22i), 

 «- (22l) which answers sufficiently to determine what it is; and the result 

 thus obtained is fully sustained by good measurements, as will be 

 seen in the table. This plane is distinctly present on one side only 

 of jB/ on the other side its presence is barely indicated. Its place 

 here is taken by a well polished and conspicuous plane *, which is 

 another striking instance of the peculiar nature of this species ; its 

 position is indicated in the sphere of projection,* and the angles on 

 e^ are given in the table. The index was calculated for each pair of 

 measurements 38° 14', 39° 24 and 4'^ 31, 6° 31 (see in the table), and 

 the results obtained were identical. From the first pair of measured 

 angles 'i a.B was found to be 34° 30^' and from the second * a-S= 

 34° 31' (required 34° 32' 38"). The index obtained was 60-f^ or 

 60-38-1, and abnormal as it certainly is, it expresses the exact posi- 



* In the projection * is placed incorrectly in the negative quadrants ; it should be in 

 the positive with + a, + n^, + r, etc. 



