44 Mr Haidinger on the Crystalline Forms 



general disposition takes place also in the present instance, 

 the portions of the two crystals, similarly situated, being 180° 

 distant from each other, compared to the plane of composi- 

 tion. 



Irregular composition is very common in this species ; it is 

 either granular, or columnar. The latter occurs much more 

 frequently. 



Observations. 



Few species in mineralogy have been so incorrectly described 

 as the ores of manganese, and, in particular, the most common 

 one among them, the prismatoidal manganese-ore. It is not 

 alone that the slight difference in the angles of two of the prisms, 

 and the situation of the perfect cleavage, was not exactly refer- 

 red to constant positions, but also colour, streak, hardness, speci- 

 fic gravity, and other important properties, were either incor- 

 rectly stated, or confounded with those of other species. The 

 insufficiency of Hauy's descriptions was felt by many mineral- 

 ogists, and several of them have endeavoured to substitute bet- 

 ter ones in their place. The result, obtained by M. Von Leon- 

 hard,* is by no means more satisfactory than that of Haiiy ; Mr 

 Phillips,-)- with his usual skill in crystallographic observations, 

 has succeeded much better. The description of the forms 

 given by Mohs £ agrees very nearly with the latter, at least 

 much more so than any two other descriptions. There are 

 some differences, however, in regard to the absolute measure- 

 ment of the angles, and in the statement that, according to 

 Mohs, the cleavage parallel to the short diagonal of the prism 

 P + ao = 99°40' is more distinct, and more easily obtained 

 than any other cleavage of the species ; whereas, according to 

 Phillips, the crystals " cleave readily, and with brilliant sur- 

 faces parallel to the lateral planes of a rhombic prism of 100° 

 and 80°, and both its diagonals." Though, in many varieties, 

 the cleavage parallel to the long diagonal of that prism may 

 in fact be obtained, it is always less distinct than that pa- 

 rallel to the short diagonal, and often not at all observable. It 



" Handbuch der Oryclognosie, p. 371. 

 t Elem. Introd. to Mineralogy, p. 243- 

 + Treatise on Mineralogy, vol. ii. p. 419. 



