and Properties of the Manganese Ores. 43 



P — co • Fracture uneven, surface, of the vertical prisms 

 streaked parallel to their common edges of intersection ; Pr 

 streaked parallel to the edges of combination with P ; P — <x> 

 parallel to those with Pr. In general, the faces are smooth, 

 and possess pretty high degrees of lustre. 



Lustre, imperfect metallic. Colour, dark brownish-black, 

 inclining to iron-black. Streak, reddish-brown. Opaque, in 

 larger masses. When broken or cleaved in the direction of 

 Pr-|-ao , and exposed to the light of the sun, minute splinters 

 are often observed, which, by transmitted light, appear of a 

 bright brown colour, so that the mineral cannot be said to be 

 absolutely opaque. 



Brittle. Hardness = 4.O.— 4.25. Sp. gr. ip 4.328, of a 

 number of fragments of crystals; = 4.312, in another experi- 

 ment of a single crystal of considerable size. 



Compound Varieties. Twin-crystals, formed in two differ- 

 ent manners. In the first of them the axes of the two indivi- 

 duals are parallel, dependant on the hemi-prismatic character 

 of the combinations of the species ; in the second, they are in- 

 clined. 1. Face of composition, parallel to Pr-f-co , axis of re- 

 volution perpendicular to it. Fig. 6. If we did not give atten- 

 tion to the compound state of this variety, shown in the present 

 instance bv the groove along the place of junction, which is 

 not always visible, we might be induced to believe that it 

 possesses a hemi-prismatic character, referred to an axis in- 

 clined upon the base of the fundamental pyramid, which 

 is not the case. One can generally trace the peculiar dis- 

 position of the crystalline faces upon each of the individuals. 

 A repetition of this law produces thick prisms, terminat- 

 ed perpendicularly upon their axis by a rough face, which 

 consists of the apices of numerous individuals, or rather of 

 numerous particles of two individuals, alternating with each 

 other. Such faces are not uncommon in the prismatoidal 

 manganese ore. 2. Axis of revolution perpendicular, face of 



fly 



composition parallel to a plane of Pr. Fig. 7. The disposi- 

 tion of the faces marked c, upon which the hemi-prismatic 

 character ef the species depends, is such, 'that a mere revolution 

 of 180° is not sufficient to bring the two individuals in the 

 position required for joining in a regular twin ; though the 



