the Ores of Manganese. 225 



portions of the two crystals similarly situated being 180 distant 

 from each other, compared to the plane of composition. This 

 peculiarity of the twin-crystals, as Professor GUSTAVUS ROSE re- 

 marks, may be shortly explained, by considering that of the he- 

 mihedral form c ; the inverse of what is found in one of the in- 

 dividuals, occurs in the other. 



Irregular composition is very common in this species : it is 

 either granular or columnar. The latter occurs much more fre- 

 quently. 



Observations. 



The name of Manganite, proposed for this species, is formed 

 in allusion to the metal which it contains, in preference to others, 

 as it is the one which occurs most frequently in nature. In 

 most mineralogical works, the characters of manganite and of 

 pyrolusite have been confounded with each other, or rather a 

 medley of the two, neither of them exactly ascertained, was given 

 as the description of a single species. The insufficiency of the 

 descriptions of HAUY and older authors was felt by many mine- 

 ralogists, and several of them endeavoured to substitute better 

 ones in their place. The result, obtained by Mr VON LEON- 

 HARD, in the first edition of his system, is by no means more 

 satisfactory than that of HAUY ; 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 de- 

 scriptions. There are some differences, however, in regard to 

 the absolute measurement of the angles, and in the statement 

 that, according to MOHS, the cleavage parallel to the short dia- 

 gonal of the prism P + oo = 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 surfaces parallel to the lateral planes of a rhombic prism 



