K S. Damt — Chan drodite from, the TilU/-Fostei' Iron Mine. 95 



nloiie gave accurate results; on it tlie angle of the same plane with 

 6"(|-?=i206) was deterniined with equal care. The results were : 

 4° 55' for the angle with ea; and hence 25° 59' with G. 



The agreement with the angles given above is as close as could be 

 desired. In the other case, the rather rare plane B (^-^r=100) was pres- 

 ent ; the crystal was minute, however, and the determination only 

 approximate. It was found that the normal to the axial plane made 

 with B an angle of 65°-V0°, and hence with the normal to the basal 

 plane 20°-25°. 



With so ample confirmation the point made cannot be even ques- 

 tioned, and it remains to reconcile it with the crystallographic proper- 

 ties of the species. It will be seen at once that the position of the optic 

 axes is totally at variance with the accepted orthorhombic character 

 of the crystals ; but it conforms to the rule for monoclinic crystals, as 

 one axis of polarization is normal to the plane of symmetry C, and 

 the others lie in it, or in other words, the optic-axes lie in a plane per- 

 pendicular to the axis of symmetry. The angles measured and cal- 

 culated, given in the various tables, show that the variation from the 

 rectangular type, if it really exist, must be very slight, as the agree- 

 ment between the angles measured and those calculated on the 

 assumed prismatic basis is very close — it being remarked that some con- 

 siderable variations in the angles given in the tables are amply ex- 

 plained by the imperfection of the crystals. Note the angles measured 

 for m^/\m^ on the twin crystal described on page 18. It was not to 

 be expected that the variation in the optical character of the crystals 

 would be so decided in view of the slight divergence which is possi- 

 ble in the crystalline form. I reserve for the future the careful re- 

 vision of the angles of this species, when I shall hope to be able to 

 command a more abundant supply of satisfactory material. It may 

 be added that the hemihedral character of the second and third types 

 of humite long ago suggested the idea that they were oblique inform ; 

 but all the crystallographic investigations thus far have seemed to 

 deny this. In the Mineralogy of Brook and Miller, the form is made 

 oblique, but this seems to be due to a misunderstanding of the planes 

 occurring on the crystals. 



It would have been interesting to extend these observations to the 

 two remaining types, but the material did not allow of it. It was 

 also desired to investigate the same subject for humite, but, though 

 some good specimens are to be found in the Yale cabinet, there were 

 no satisfactory crystals to be had, and the matter is left for others, 

 who have a larger choice of specimens. The axes as already men- 

 tioned do not appear distinctly except in oil ; in the first-mentioned 



