188 Mr. L. Fletcher's Crystallographic Notes. 



tude of the faces being approximately in descending order: — 



(100) (210) (Oil) (115) (103) (315) (110) 

 (311) (112) (001) (101) (010) (111) (113) 

 (114) (117) (131) (313). 



Of these forms, (1 3 1), (3 1 1), (1 1 7) were first observed on 

 the crystals from Oker ; (2 1 0), (0 1 1), (1 1 2) are old forms, 

 which did not occur on those crystals ; while (1 1 4) has not 

 been previously described : the faces of the latter form were 

 about the same size as those of the forms (1 1 3), (1 1 7). The 

 following measurements were obtained by the method of maxi- 

 mum illumination : — 



Observed. Calculated. 



115.114 ..60 5° 55 



113.114 ..80 88 



VI. Nagyagite. 

 This mineral, which never seems to present very perfect 

 crystals, has been long assumed to belong to the tetragonal 

 system. Lately Schrauf* has pointed out that the crystals he 

 has examined present a rhombic rather than a tetragonal deve- 

 lopment of planes; while from the measurement of angles of the 

 crystals, and of the striations upon the basal plane, he thinks 

 that the crystals really belong to even a lower type of sym- 

 metry than the rhombic. Assuming, however, rhombic axes, 

 he gives the following indices of observed planes : — 



,(010) (16 0) (13 0) (12 0) (0 51) (0 31) (011) 

 (252) (121) (343)* (111). 



All the planes, with the exception of the basal plane (0 1 0) 

 are almost linear. He measured five crystals, and found that 

 in three (1 2 0) was associated with (0 1 1), whilst tetragonal 

 symmetry would have required (0 21); in a fourth (12 0) is 

 associated with (0 31), while the fifth presented no measurable 

 planes in these zones. In one, and only one, of these crystals 

 (1 3 0) and (0 5 1) occur ; (121) occurs on three crystals, 

 (1 1 1) on two, (2 5 2) and (3 4 3) each on a single crystal. 

 From the above, Schrauf hopes that it may be possible to di- 

 stinguish between the two domes. 



To the above forms may be added (1 1 0), (1 3 1), (1 4 1). 



Six crystals were measured; and of these five were removed 

 from the same specimen; and all came from the old locality, 

 viz. Nagyag in Transylvania. The following angles were 



* Groth's Zeitschrift fur Krystalhgraphie, 1878, p. 239. 



