556 WATSON AND WHERRY: PYROLUSITE FROM VIRGINIA 



mony with the view that the crystals and crystalline pyrolusite 

 from Virginia are of pseudomorphous origin; these features may 

 pertain to original pyrolusite. The writer is fully aware of the 

 prevailing view that pyrolusite crystals are pseudomorphous in 

 origin, and that many pyrolusite crystals have the form of man- 

 ganite. The present study, however, of the material from the 

 Virginia locality suggests the probability that the pyrolusite 

 may be original; that is, it has a distinct crystalline form of its 

 own, and is not secondary in the sense of being derived from 

 manganite by dehydration and oxidation. More extended in- 

 vestigation of the manganese oxide minerals, especially pyrolu- 

 site and manganite, is under way by the writer, with the hope 

 that more definite conclusions may be reached as to their genetic 

 relations. 



PART II. CRYSTALLOGRAPHY 



The specimens examined contain irregular cavities a centi- 

 meter or two in diameter, lined with brilliant crystals corre- 

 sponding in form to manganite,^" averaging about a millimeter 

 across. The habit varies from tabular, owing to the predomi- 

 nance of the front pinacoid or of prisms vicinal to it, to wedge- 

 shaped, from the combination of prisms and side-domes. Sub- 

 parallel intergrowth is frequent, causing variations of half a degree 

 or so in the angles, so that comparatively few of the crystals 

 can be used for measurement. The crystals are attached to the 

 m.atrix in the general direction of the side pinacoid, and are 

 doubly terminated as far as the vertical axis is concerned. The 

 termination is of a type very unusual in manganite, being pro- 

 duced chiefly by side-dome faces, of which five prove to be new. 

 Single faces of two possibly new pyramids, and of one previously 

 ncted pyramid, are also present, one on each of three crystals. 



The development toward the attached end of axis b is very 



^^ For the sake of simplicity the crystals will be called manganite, even though 

 their physical and chemical properties are those of pyrolusite. 



