of Mineral Species. 99 



tinues, in those parts whose streak is black ; nay, it seems to be 

 more easily obtained, particularly the faces parallel to the short 

 diagonal of the prism of 99 40'. From chemical considerations, 

 Professor LEOPOLD GMELIN had formed nearly the same opi- 

 nion in regard to a change of composition within the crystals or 

 crystalline masses of one of the species. One of them is a hy- 

 drate of the oxide of manganese, and that is the prismatoidal 

 manganese-ore, giving a brown streak : the other is the hyper- 

 oxide, formed by loss of water and absorption of oxygen, and it 

 gives a black streak. Hitherto no crystals of the latter substance 

 have been described, that did not depend upon the previous ex- 

 istence of the prismatoidal manganese-ore. Professor GUSTAVUS 

 ROSE of Berlin shewed me small crystals, having the form of right 

 rhombic prisms, with their acute lateral edges replaced, and mea- 

 suring 86 20' and 93 40', a prism not to be found in any of the 

 known varieties of the former species. But the faces not being 

 very bright, and the measurements therefore not quite decisive, 

 inferences drawn from the observed difference in the angles might 

 prove erroneous. 



The pyramidal manganese-ore, too, sometimes appears to be 

 a product of the decomposition of the prismatoidal species. In 

 a specimen in Mr ALLAN'S cabinet, the pyramidal species forms 

 very distinctly the substance of elongated crystals, resembling 

 those of the latter ; but unfortunately the decomposition has 

 proceeded so far, that the surface of the original crystals no 

 longer exists, in a manner similar to what occurs in several in- 

 stances of malachite in the shape of blue copper. We cannot 

 guess at the chemical change taking place here, as the composi- 

 tion of the pyramidal manganese-ore is entirely unknown. From 

 the preference given to the varieties with a black streak above 

 the pyramidal species by the miners of Ihlefeld, where Professor 

 GUSTAVUS ROSE last summer found the pyramidal species to oc- 

 cur in a particular vein in porphyry, it would appear that this 



N 2 



